Multilocation video conference terminal including controllable conference location reconfiguration

ABSTRACT

In a multilocation video conference system, conference locations are dynamically added to or deleted from an ongoing conference without disrupting it. This is realized by controllably enabling or disabling communications ports assigned to the conference locations in a Multilocation Conference Terminal (MLCT). Both audio information and control channel information transmitted from a conference location assigned to a port are monitored to determine if the audio information is satisfactory and if the control channel communication is &#34;good&#34;. If the audio information is unsatisfactory, it is muted and selection of the video from that location as a one-and-only-one talker is inhibited. If the control channel communication is not good the video signal from the conference location is excluded from the conference.

RELATED APPLICATIONS

U.S. patent applications Ser. No. 545,206 (J. R. Colton et al Case10-2-1), Ser. No. 545,461 (J. R. Colton et al Case 12-4-3), Ser. No.545,363 (D. Scordo Case 5), Ser. No. 545,364 (D. Scordo Case 7), andSer. No. 545,162 (D. Scordo Case 8) were filed concurrently herewith.

TECHNICAL FIELD

This invention relates to audio-visual conferencing systems and, moreparticularly, to a system for controllably connecting a plurality ofconference locations together including controllable reconfiguration oflocations in a conference.

BACKGROUND OF THE INVENTION

In known video conference arrangements addition or deletion of locationsto or from an ongoing conference tended to cause disruptions in thevideo and audio information being transmitted to the conferencelocations. Additionally, there was no monitoring of the video or audioinformation of the locations to be added to the conference or of any ofthe conference locations except that provided by human viewers or theconferees themselves. Consequently, if the location to be added or alocation already connected in the conference has unsatisfactory audio orvideo signals, undesirable disruptions in the conference can occur.

Moreover, when adding or deleting one or more locations to or from,respectively, an ongoing conference, disruptions could still occur inprior arrangements because the existing transmission connections usuallyneeded to be physically reconfigured. The resulting disruptions can beof a momentary nature or for long periods of time. Indeed, the locationhaving unsatisfactory audio or video signals causing the disruptionswould usually require physical removal from the conference.

SUMMARY OF THE INVENTION

Additions or deletions of locations to or from, respectively, an ongoingconference are realized, in accordance with an aspect of the invention,without disrupting the conference, by controllably enabling ordisabling, respectively, communication ports assigned to the individuallocations in a multilocation conference terminal (MLCT). The ports arenot enabled until the MLCT establishes "good" communications with thelocations and provides the locations with necessary status informationregarding the ongoing conference. The locations in the conference arecontinually monitored, and when the "good" communications is lost thecorresponding location is deleted from the conference by disabling itsassigned MLCT port.

More specifically, when an MLCT port is assigned a location to bedynamically added to the conference, the audio signal from that locationis monitored to determine if it is satisfactory and a control channel toand from the location is monitored to determine if the "good"communication exists. If the audio information is unsatisfactory, theaudio from that location is muted and, therefore, does not affect theconference audio information. Additionally, the video signal from thatlocation is also inhibited from being the location selected as having aone-and-only-one talker for transmission to the other locations. If thecontrol channel communication from the location is not "good" the videosignal from that location is also excluded from the video to betransmitted to the other locations in the conference.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription of an illustrative embodiment taken in connection with theappended figures in which:

FIG. 1 shows in simplified block diagram form a system arrangementincluding a Multilocation Conference Terminal (MLCT) incorporating anembodiment of the invention;

FIG. 2 depicts the VTS signal data format used in the system of FIG. 1;

FIG. 3 illustrates the cross connect frame format used in the system ofFIG. 1;

FIG. 4 shows in simplified block diagram form details of microprocessorsystem 101 employed in the system of FIG. 1;

FIG. 5 depicts in simplified block diagram form details of transmissionfacility interface 104 used in the system of FIG. 1;

FIG. 6 shows in simplified block diagram form details of cross-connect102 employed in the system of FIG. 1;

FIG. 7 illustrates in simplified block diagram form details of timemultiplexed switch 601 used in cross-connect 102 of FIG. 6;

FIG. 8 shows in simplified block diagram form details of NxN switch 602used in cross-connect 102 of FIG. 6;

FIG. 9 illustrates the cross-connect 102 audio and video time slotassignments utilized in effecting an aspect of the invention;

FIG. 10 shows in simplified block diagram form details of bridge 103used in the system of FIG. 1;

FIG. 11 depicts in simplified block diagram form details of bridge inputcircuit (BOC) 1001 used in bridge 103 of FIG. 10;

FIG. 12 illustrates the output frame format of the audio encryptionprocess which is useful in describing aspects of the invention;

FIG. 13 illustrates timing diagrams useful in describing the encryptionand decryption process;

FIG. 14 shows in simplified block diagram form details of audiodecryptor (DEC) 1002 used in bridge 103 of FIG. 10;

FIG. 15 illustrates a flowchart of the decryptor program routine for thecommon control operation of decryptor 1002 to controllably decrypt theincoming audio and control information of bridge 103;

FIGS. 16, 17, 18 and 19 when connected as shown form a flow chart of thePADEC sub-routine used in the decryptor routine of FIG. 15;

FIGS. 20 and 21 connected as shown form a flow chart of the CENCHRsub-routine used in the decryptor routine of FIG. 15;

FIG. 22 shows in simplified block diagram form details of audio bridge1003 employed in bridge 103 of FIG.10;

FIGS. 23, 24, 25 and 26 when connected as shown form a flow chart of aprogram routine for controllably operating DSP 2201 in audio bridge 1003of FIG. 22;

FIGS. 27, 28, 29 and 30 when connected as shown form a flow chart of aprogram routine for controllably operating DSP 2202 in audio bridge 1003of FIG. 22;

FIG. 31 is a flow chart of a DSP control (DSPCTL) program task foroperating host processor 2203 to interact with digital signal processors(DSPs) 2201 and 2202 in audio bridge 1003 of FIG. 22;

FIG. 32 is a flow chart of a noise guard program (NGRD) sub-routine usedin the DSPCTL program task of FIG. 31;

FIG. 33 shows in simplified block diagram form details of audioencryptor (ENC) 1004 employed in bridge 103 of FIG. 10;

FIGS. 34, 35 and 36 when connected as shown form a flow chart of programroutine ESCVC to effect common control of the mixed audio informationfrom audio bridge 1003;

FIG. 37 is a flow chart of the PAENC sub-routine used in the ESCVCprogram routine of FIGS. 34, 35 and 36;

FIG. 38 depicts in simplified block diagram form details of bridgeoutput circuit (BOC) 1005 used in bridge 103 of FIG. 10;

FIG. 39 shows in simplified block diagram form details of controlprocessor (CP) 1006 used in bridge 103 of FIG. 10;

FIG. 40 is a flow chart of program routine XPCTL for operating controlprocessor 1006 of FIG. 39 to control communications to audio decryptor1002, audio bridge 1003 and audio encryptor 1004;

FIG. 41 is a flow chart of sub-routine PREST used in the XPCTL programroutine of FIG. 40;

FIGS. 42, 43, 44 and 45 when connected as shown form a flow chart of thePRNLA sub-routine employed in the XPCTL program routine of FIG. 40;

FIG. 46 is a flow chart of the PRDENT sub-routine employed in the XPCTLprogram routine of FIG. 40;

FIG. 47 illustrates the relationship among the program tasks thatperform the video selection algorithm and the fast update algorithm;

FIGS. 48, 49 and 50 when connected as shown form a flow chart of stepsperformed in the new talker process (NTP) task by control processor 1006of FIG. 39;

FIG. 51 is a flow chart of the CONTEND sub-routine used in the NTPprogram task of FIGS. 48, 49 and 50;

FIG. 52 is a flow chart of the NTSAVE sub-routine used in the NTPprogram task of FIGS. 48, 49 and 50;

FIGS. 53, 54, 55 and 56 when connected as shown form a flow chart of thesteps performed in the new graphics process (NGP) program task bycontrol processor 1006 of FIG. 39;

FIG. 57 is a flow chat of the GRACK sub-routine used in the NGP programtask of FIGS. 53, 54, 55 and 56;

FIGS. 58 and 59 when connected as shown form a flow chart of the stepsperformed in the video selection (VSEL) program task by controlprocessor 1006 of FIG. 39;

FIGS. 60 and 61 when connected as shown form a flow chart of the stepsperformed in the fast update generator (FUGEN) program task by controlprocessor 1006 of FIG. 39;

FIGS. 62 and 63 when connected as shown form a flow chart of the stepsperformed in the fast update monitor (FUMON) program task by controlprocessor 1006 of FIG. 39;

FIGS. 64 and 65 when connected as shown form a flow chart of the stepsperformed in the video switch (VSWCH) program task by control processor1006 of FIG. 39; and

FIG. 66 is a flow chart of the SWITCHLK sub-routine used in the VSWCHprogram task of FIGS. 64 and 65; and

FIG. 67 and 68 when connected as shown form a flow chart of stepsperformed in facility administrator (FADMIN) program task by controlprocessor 1006 of FIG. 39.

DETAILED DESCRIPTION

General

The Multilocation Conference Terminal, hereinafter referred to as MLCT,is employed to establish and control a multilocation audiovisualteleconference in a Video Teleconferencing Service, hereinafter referredto as VTS, for conferences involving a plurality of VTS conferencerooms. Accordingly, FIG. 1 shows in simplified block diagram form asystem arrangement including a plurality of MLCT's, namely, MLCTs 1through Y and a plurality of conference rooms, namely, rooms 1 throughM, which may be controllably interconnected via the MLCT's andtransmission facilities 10. Each of MLCTs 1 through Y includes aplurality of VTS communication ports, associated with data links 106-1through 106-N, for interconnecting up to N rooms via transmissionfacilities 10. Transmission facilities 10 include both terrestrialcommunication channels and/or satellite channels. Additionally,facilities 10 also include arrangements for interconnecting one or moreof the conference rooms, i.e., conference locations, to an associatedMLCT for establishing a conference. These connections may be establishedmanually or via switching apparatus, for example, a digital crossconnect arrangement. When satellite channels are deployed a multiplicityof MLCTs are used to reduce the number of end-to-end satellite hops to amaximum of one. In the satellite mode of operation, the MLCTs form adistributed conferencing control structure.

A number of conference configurations are realizable including one ormore MLCTs. For example, a single MLCT may be used to include up to N=6rooms in a conference over either local or long-haul terrestrialcommunication facilities. While it is possible to use only one MLCT inan all terrestrial conference, it is not necessary to do so. Two MLCT'scan be connected terrestrially, allowing savings of transmissionfacilities. In a mixed satellite/terrestrial conference network,distributed conferencing is used with a MLCT being associated with eachsatellite earth station. Satellite and terrestrial connections remainunchanged during an ungoing conference.

Each of MLCTs 1 through Y provides a time-space-time switching functionand includes microprocessor system 101, digital cross connect 102,bridge 103, transmission facility interface 104 and a plurality of dataterminal ports 105-1 through 105-T. Data terminals may be connected todata terminal ports 105-1 through 105-T for manually providingteleconference setup information including VTS port assignment and otherinformation needed for the ongoing conference. Where an operationssupport system or other reservation service computer is available, thedata terminal ports are used to connect the MLCT thereto for carryingout the necessary conference setup and maintenance procedures.

As described hereinafter each MLCT provides audio mixing and videoswitching of encrypted digital VTS signals which are supplied via VTSports and data links 106-1 through 106-N, respectively, to transmissionfacilities 10 and, in turn, to associated ones of rooms 1 through Mand/or to another MLCT. In this example, each MLCT includes six (6) VTSports, i.e., N=6.

Each of conference rooms 1 through M includes known audio and videoequipment. For example, each room includes video monitors and cameras120, speakers and microphones 121, audio detectors 122, pictureprocessor 123, room controls 124 and room controller 125. Roomssubstantially identical to rooms 1 through M are presently commerciallyavailable for point-to-point audiovisual conferences. Room controller124 is arranged to respond to an auxiliary control channel signal in theVTS signal from an associated MLCT for controlling picture processor 123in accordance with aspects of the invention as will be described below.Picture processor 123 is commercially available from Nippon ElectricCorporation and is described in Western Electric Company KS-22456specification. U.S. Pat. No. 3,601,530 discloses a video conferencearrangement using voice-switched cameras in a conference room. Furtherdetails of the point-to-point conference room are broadly described inan article by Mr. Bernard A. Wright entitled, "The Design and Provisionsof PICTUREPHONE Meeting Service (PMS) Conference Centers for VideoTeleconferencing", in the IEEE Communications Magazine, pages 30-36,March 1983. Also see an article by Audrey J. Kames and W. Gordon Heffronentitled, "Human Factors Design of PICTUREPHONE Meeting Service" inGLOBECOM '82 Conference Record, Volume 3, pages 976-981, November 1982for a broad overview of the commercially available point-to-point videoteleconferencing service.

As indicated above, the MLCTs are interconnected via transmissionfacilities 10 to associated ones of rooms 1-M and/or another one of theMLCTs through VTS ports 501-1 through 501-N (FIG. 5) and data links106-1 through 106-N, respectively. The VTS signals are transmitted overone or more time division multiplexed channels, for example, T1 lines,depending on the data rate being employed. In this example, the datarates are selectable at 1.544, 3.088 or 6.176 Mb/sec. In a specificembodiment, two T1 lines are used having the 3.088 Mb/sec data rate witheach T1 line having an incoming data link and an outgoing data link. TheVTS data format is shown in FIG. 2. One of the T1 lines, designated DS-1No. 1 carries encoded audio and control channels in addition to videodata. The other T1 lines, designated DS-1 No. 2 through DS-1 No. 4 carryonly video data. The T1 lines do not have standard DS-1 formats exceptfor framing bits Ft.

The format for DS-1 No. 1 is a combination of three groups ofinformation; one associated with the video signal; another with audio;and another with the control data channel. The control group consists ofa single C-bit per frame located in word 1 bit 4, as shown in FIG. 2.The C-bit implements an 8 Kb/sec data link between the MLCT and theremote location communicating to it, i.e., either a room or anotherMLCT. The audio group consists of 16 audio bits, A1 to A16, a D-bit, aswell as "1"s in the shown bit positions, all within the first threewords of DS-1 No. 1. The A-bits represent two samples per frame ofmu-255 coded audio. Bits A1 through A8 are audio bits belonging to afirst sample of the audio signal. Bits A9 through A16 are audio bitsbelonging to the second sample of the audio signal. The audio may beencrypted or transmitted "on the clear". The D-bit is used forsynchronization of the audio encryptor 1004 and decryptor 1002 describedbelow.

The first three words of the DS-1 No. 1 contain fixed "1"s in the shownposition to maintain the required "1"s density during these words forall possible codes transmitted. The remaining bits in DS-1 No. 1, i.e.,FD, X, X, and those of DS-1 No. 2 through DS-1 No. 4 contain videoinformation. The MLCT does not process these remaining bit locationsaside from maintaining their relative position throughout its switchingfunction. Indeed, except for the information included in the C-bitposition, the VTS data format is essentially identical to that employedin the existing point-to-point teleconferencing service availablethrough American Telephone and Telegraph Company.

MLCT Operation-General

A Multilocation Conference Terminal (e.g., MLCT1, FIG. 1) located in aVideo Teleconferencing Service (VTS) is connected via transmissionfacilities 10 to conference locations, i.e., rooms and other MLCTsdependent on the conference being set up.

The MLCT is always in one of the following modes:

Standby (SBY): In the standby mode the MLCT is not processing aconference. It is continually performing internal diagnostics to ensurethat it is fully operational.

Sender's Choice (SC): A MLCT in the SC mode is set up to operate a SCconference.

Viewer's Choice (VC): A MLCT in the VC mode is set up to operate a VCconference.

No MLCT mode changes are allowed while the MLCT is processing aconference. To change modes the MLCT must first be removed from theconference.

The steps to set up the MLCT for a conference are as follows:

1. Enter command(s) into the MLCT through an input terminal connected toone of input ports 105-1 through 105-T to place it in the desired modefor the conference (VC,SC) and to define the MLCT's ID in theconference.

2. Setup transmission connections to the MLCT required for theconference.

3. Enter command(s) into the MLCT to assign the VTS ports as either:

a. connected to a room (specify room ID).

b. a broadcast link (outgoing transmission path to all other MLCTs inthe conference) and/or a MLCT link (incoming transmission path fromanother MLCT, specify that MLCT's ID).

Once these steps are completed the conference is in progress and theMLCT will automatically set up and maintain communication with itsserving rooms, i.e., rooms associated to it, and other MLCTs.

If communication over a VTS port connected to a room, e.g., room I, islost (or is never established in the first place) the MLCT operates theconference without room I. No other room can view room I. The audiotransmitted by room I is not muted unless audio decryptor 1002 (FIG. 10)detects a facility failure in the audio signal incoming from room I. Thevideo transmitted to room I is selected by assuming that room is in theautomatic video switching (AVX) mode. Thus, room I essentially becomes aspectator. Room I views a new talker room or a new graphics room asdescribed below. The other rooms hear room I if its audio signal is notmuted, but they cannot view room I.

If communication over a broadcast link or MLCT link is lost (or is neverestablished in the first place) the MLCT operates a conference for onlythe rooms that it serves.

Configuration changes to a conference already in progress are realizedas follows:

1. If any of the MLCT's room, broadcast, or MLCT links are to beremoved, then:

a. Command(s) must be entered into the MLCT that unassign those links.

b. The port connections to be removed are taken down.

2. If any room, broadcast, or MLCT links are to be added to theconference, then:

a. The port connections just assigned are set up.

b. Command(s) must be entered into the MLCT to assign each new port asconnected to a room or as a broadcast link and/or MLCT link.

Any number of configuration changes can occur during a conference.

The microprocessor system (MS) 101 (FIG. 1) maintains control of theMLCT. Commands are entered through the links 105-1 through 105-T intothe MS 101. The MS 101 interprets these commands and issues signals toset up the other components of the MLCT. The MS 101 is responsible fordownloading the proper program code into bridge 103 when any MLCT modechange is performed and for informing the bridge on all the conferenceattributes (MLCT's ID and link assignments). MS 101 cannot communicatedirectly with other MLCTs or any room controllers.

Bridge 103 computes the weighted sums of the audio signals, receives thecontrol signals that come over the incoming links from the rooms andother MLCTs, generates the control signals that are transmitted to therooms and other MLCTs, signals MS 101 to execute video switches, andmonitors the links to detect facility failures in the control and audiosignals.

MS 101 and bridge 103 interact during a conference to perform twoprimary functions. Bridge 103 directs MS 101 to perform switching of thevideo signals. MS 101 and bridge 103 keep each other informed of linkfailures.

Cross-connect 102 and transmission facility interface 104 provide bridge103 with audio and control signals, receive conferenced audio andcontrol signals from bridge 103 and perform the switching of the videosignals under the control of the MS 101.

The MLCT can be in one of three modes (SBY, SC, or VC) as describedabove. Bridge 103 can be in one of four states. Three of the bridgestates correspond to the MLCT modes (SBY, SC, and VC). Bridge 103 entersthe fourth state, called the kernel state, while MS 101 is downloading anew program code. The MLCT's mode and bridge state are set up prior tothe start of the conference and do not change while the conference is inprogress. By definition the MLCT mode and bridge state during aconference must be the same, either sender's choice or viewer's choice.

When the conference is first initiated, bridge 103 comes up in the standalone (SA) condition. In the SA condition bridge 103 operates aconference for just its serving rooms.

When the bridge verifies that all of its broadcast links and MLCT linksare active, it assumes the distributed (DS) condition. In the DScondition the bridge will operate in the conference with other MLCTs.The conference messages are exchanged with the other MLCTs. If the MLCTis in the DS condition and one of its broadcast or MLCT links becomesinactive, bridge 103 assumes the SA state again.

If the bridge has communication with the other MLCTs it will notify themof its DS/SA condition transitions by transmitting an MLCT status(MSTAT) message specifying the condition (DS,SA) it has assumed.

The MLCT will try to operate the conference as set up by the commandsentered through MS 101. At the initialization of the conference the MLCTmode (VC,SC) and MLCT's own ID is specified before any links areassigned. MS 101 will have already downloaded the program code for therequired bridge state into bridge 103. Bridge 103 starts up in the SAcondition in the mode specified (VC,SC) with all ports unassigned.

1. MS 101 communicates with bridge 103 to assign a port to a room (MSspecifies room ID) or assign a port's in-link, i.e., an incomingtransmission path, as a MLCT link (MS specifies MLCT ID) and/or assignthe port's out-link, i.e., an outgoing transmission path, as a broadcastlink.

2. Bridge 103 marks the in and/or out link as assigned, inactive, andmuted.

a. If the port is assigned to a room bridge 103 marks that roomassigned.

b. If the in link is assigned to an MLCT bridge 103 marks that MLCTassigned.

3. Bridge 103 trys to establish communication over the links. Ifcommunication is established the link is marked active.

4. If decryptor 1002 (FIG. 10) detects no facility failure in the audiosignal of the assigned incoming link, bridge 103 unmutes the audiosignal and marks the in link as unmuted.

Any subsequent command entered through one of the MLCT's administrativelinks that declares link(s) to be assigned will be accepted only if itdoes not reassign links that are presently assigned. All of thesesubsequent commands are executed (if they are accepted) in the samemanner as the first command that assigned links in the conference.

Any subsequent command entered through one of the MLCT's administrativelinks that declares an in link and/or out link to be unassigned will beaccepted only if that in link and/or out link is presently assigned. MS101 communicates with bridge 103 to update the bridge's linkassignments. Bridge 103 does the following:

1. Mark the specified links as unassigned.

2. If any links are being marked unassigned were assigned to rooms, markthose rooms as unassigned.

3. If the last MLCT link assigned to another MLCT, MLCT J, is beingmarked unassigned, then:

a. MLCT J is marked unassigned.

b. All rooms served by MLCT J are marked unassigned.

Any link or room that is marked unassigned is automatically markedinactive and muted.

While the MLCT is involved in a conference (i.e., some links areassigned), no command entered into the MLCT that specifies changingeither the MLCT mode or MLCT's own ID will be accepted.

Broadcast links are dedicated to ensure that the MLCT will be able totransmit any of the required combination of video signals to the otherMLCTs. At the time of the conference set-up, the MLCT decides how todedicate each of its broadcast links. The dedication of these broadcastlinks will not be changed unless the conference set up is changed.

In a Viewer's Choice conference, MLCTs may need more than one broadcastlink. The push-to-see (PTS) mode requires that MLCTs be able to transmitany combination of video signals to the rooms. In a Viewer's Choiceconference the broadcast links are dedicated according to the followingpriorities:

1. If the MLCT has enough broadcast links it assigns a broadcast link toeach of its serving rooms. In this case all of the other MLCTs alwaysreceive video from every room served by this MLCT.

2. If the MLCT has enough broadcast links it assigns a broadcast link toeach of the rooms served by another MLCT. In this case the MLCT willalways be able to transmit the video required for each of the roomsserved by other MLCTs.

3. If the MLCT does not have enough broadcast links to satisfy either oftwo previous cases, then the MLCT does not have enough broadcast linksto operate properly in the conference. In this case the MLCT prints awarning message. If the MLCT has more than enough broadcast links neededfor dedication to the rooms, then it marks the remaining broadcast linksas undedicated.

In a Sender's Choice conference each MLCT requires only one broadcastlink. This broadcast link is not dedicated.

Returning to MLCT FIG. 1, there is shown MLCT1 including microprocessorsystem (MS) 101, digital cross connect 102, bridge 103 and transmissionfacility interface 104 which are interconnected as shown by amultiplicity of data and control paths in order to perform requiredmultilocation audio video teleconferencing functions.

Microprocessor System (MS)

The MLCT microprocessor system (MS) 101 is shown in simplified blockdiagram form in FIG. 4. Shown are CPU 401, memory 402, back-up memory403, peripheral interface 404 and communications interface 405 allinterconnected via bus 406. Memory 402 includes program storage in ROMand data storage in RAM. Back-up memory 403 stores data pertaining toconference set-up in a nonvolatile storage such as bubble memory inorder to maintain conference set-up information integrity in the eventof a short-time power outage to eliminate the re-entry of theinformation at the data terminals. Microprocessor system 101communicates via peripheral interface 404 to transmission facilityinterface 104, cross connect 102 and bridge 103. Peripheral interface404 communicates to the transmission facility interface 102 to transfertime-slot map information as discussed hereinafter or in other words toallow a connection of a VTS port of the MLCT. Microprocessor system 101communicates through peripheral interface 404 to cross-connect 102 tomap the time multiplexed switches (TMS) for configuring the TMS outputdata links 603-1 through 603-N (FIG. 6) to contain the information shownin TABLE 3 below and to perform the video switch function by the use ofNxN switch 602 (FIG. 6) as described below. The bridge 102 communicationis also handled by the peripheral interface 404. Information transfer tobridge 103 includes VTS port assignment, conference set-up including thedesignation of the various connections to the VTS data links and whetherconference rooms or MLCTs are connected to them. Communicationsinterface 405 is used to provide a connection to serial data terminalconnections 105-1 through 105-T, in one example T=5, which are used tocommunicate with the conference set-up operator, a reservation system,or a port system to handle the conference reservation from multiplelocations and to perform maintenance operations on the system. Themicroprocessor system 101 functions may be divided into three distinctclasses: maintenance, operational (video switches), and conferenceparameter management. It performs all communication and administrativefunctions for the MLCT. It has communication access to all majorcomponents of the system to perform conference set-up operations,maintenance functions, error recovery, transmission facility performancemonitoring, and video switches upon request from bridge 103.Microprocessor system 101 is essentially identical to the arrangementdescribed by A. J. Cirillo, L. F. Horney II and J. D. Moore in anarticle entitled, "DACS Microprocessor System", NTC'81, IEEE 1981National Telecommunications Conference, Vol. 1, pages B1.3.1-B1.3.6,Nov., 1981.

Transmission Facility Interface

Details of transmission facility interface 104 are shown in simplifiedblock diagram form in FIG. 5. It includes VTS ports 500-1 through 500-N,each of which includes a plurality of digroup circuits 501-1 through501-R. As noted above, in this example two T1 lines are used for each ofVTS ports 500-1 through 500-N and, therefore, two digroup circuits areused per VTS port. Again, the VTS ports connect transmission facilityinterface 104 via transmission facilities 10 (FIG. 1) to associateconference rooms and/or to associated MLCTs. Each of digroup circuits501-1 through 501-R is connected via a corresponding one of data links502-1 through 502-R to cross connect 102. Each of digroup circuits 501-1through 501-R in a VTS port receives data from cross connection 102 on ashared data link, for example, via data link 108-1. Each digroup circuitin transmission facility interface 102 provides a read-write capabilityto microprocessor system 101 of all error source registers (not shown)and control registers (not shown). The error source registers relate tovarious internal hardware error detectors as well as to transmissionfacility related problems such as T1 framing errors, out-of-frameconditions, slips and carrier failure alarms (CFA). In addition to theDS-1 termination function, the digroup circuit implements the "time"portion of the time-space-time switching function of the MLCT. Itsreceiving section contains a line clock recovery circuit, a framer forrecovery of the Ft framing bits, an elastic store, transmission facilityerror detectors and a time-slot interchanger (RTSI) (all not shown) totransform the T1 frame format to one compatible with that of thecross-connect, i.e., the cross connect data format of FIG. 3. Itstransmitting section contains a second time-slot interchanger (TTSI)(not shown) and a DS-1 signal formatter (not shown) to convert thecross-connect data format to the DS-1 format.

The signal format provided by the digroup circuit to the cross-connect102 is shown in FIG. 3. It consists of a frame of 256 time-slots,labeled time-slot 0 through time-slot 255. It is transported by threerails at a data rate of 8.192 Mb/sec each. Each time-slot contains 12bits of information: 8 data bits, labeled D1 through D8, three sparebits, and a parity bit that provides odd parity over the 12 bits in thetime slot and the eight-bit code representing the time-slot number. TheRTSI may be configured to fill any of the 256 time slots with thereceived 24 words of the incoming T1 line. Similarly, the TTSI maychoose any of the 256 time-slots (up to 24) and selectively place themin the outgoing T1 line.

In this example, the digroup circuits 502-1 through 502-R, where R=4,for VTS port 500-1 identify and format the incoming DS-1 signals.Specifically, digroup circuit 502-1 identifies the video controlsignals, c bit and audio signals in words 1, 2 and 3 and formats them intime slots 0, 1 and 2 in the cross connect frame format of FIG. 3.Similarly, it identifies the video information in words 4 through 24 andformats it into time slots 64 through 84 in the cross connect frameformat of FIG. 3. Digroup circuit 502-2 identifies the video informationin signal DS1 No. 2 and formats it into time slots 85 through 108 of thecross connect frame format. Digroup circuit 502-3, if used, identifiesthe video information in signal DS1 No. 3 and formats it into time slots108 through 132 of the cross connect format. Finally, digroup circuit502-4, if used, identifies the video information in signal DS1 No. 4 andformats it into time slots 133 through 156 of the cross connect format.For VTS port 500-2 the only difference from VTS port 500-1 is in theformatting of the video control signal, c-bit and audio signals. In thisexample, this information is formatted into time slots 4, 5 and 6 of thecross connect frame format.

This formatting is shown in FIG. 9 relating to the cross connect 102frame formats. There is shown that for VTS port 500-3 the video control,c-bit and audio information is in time slots 8, 9 and 10, for VTS port500-4 in time slots 12, 13 and 14, for VTS ports 500-4 in time slots 16,17 and 18 and for VTS port 6, i.e., N=6, in time slots 20, 21 and 22.These formatted signals are supplied to cross connect 102.

Such digroup circuits are known in the art, see for example the digroupcircuit described by R. P. Abbott and D. C. Koehler in an articleentitled, "Digital Access and Cross-Connect System - SystemArchitecture", NTC'81, IEEE 1981 National Telecommunication Conference,Vol. 1, pages B1.2.1-B1.2.7 at page B1.2.2., Nov., 1981.

VTS ports 500 can be assigned in a conference as follows:

1. Both the input and output data links of a VTS port are assigned to aroom;

2. The output data link of the VTS port is assigned as a so-calledbroadcast link, i.e., the outgoing signal is supplied to all other MLCTsin the conference; and

3. The input data link of the VTS port is assigned as a MLCT link, i.e.,it receives an outgoing broadcast signal from another MLCT.

Cross Connect

Details of cross-connect 102 are shown in simplified block diagram formin FIG. 6. Cross-connect 102 provides the space switching function ofthe MLCT time-space-time switching function. Cross-connect 102 includestime-multiplexed switches (TMS) 601-1 through 601-N and NXN switch 602.Time multiplexed switches 601-1 through 601-N are assigned on aone-to-one basis to VTS ports 500-1 through 500-N, respectively, intransmission facility interface 104. Data links 107-1 through 107-N aresupplied from transmission interface 104 to TMS 601-1 through 601-N,respectively. Also supplied to TMS 601-1 through 601-N are data links109-1 through 109-N, respectively. Data links 109-1 through 109-N carryprocessed information from bridge 103 relating to associated VTS ports500-1 through 500-N, respectively. Each of TMS 601-1 through TMS 601-Nselects on a per time slot basis information from its input data linksand combines the information to generate at its output data link another256 time-slot composite signal frame containing the time slot selectionthat the TMS is programmed to produce. It is important to note that thevideo information from a conference location is included in severalincoming signals and that the TMS combines the information into the onecomposite signal. This combining of all the video information from acorresponding conference location into one composite signal enables, inaccordance with an aspect of the invention, a rapid switch of the videobeing viewed. Outputs from TMS 601-1 through 601-N are controllablyselected by NXN switch 602 and supplied via 108-1 through 108-N,respectively, to transmission facility interface 104 for transmission onT1 transmission facilities to transmission facility 10 to be routed toappropriate ones of the conference room and/or other MLCTs, TMS 601-1through TMS 601-N and NXN switch 602 are controlled by microprocessorsystem 101 to realize a desired video and audio switch to the ports.Through this arrangement anyone of the composite signals from TMS 601-1through TMS 601-N is selectable via switch 602 for transmission to anyone of VTS ports 500.

It is noted that each VTS port receives an audio mix from all the otherports. Thus, the audio that must accompany the video to each port is adifferent mix. The appropriate audio mix and video are switched to theVTS ports by employing cross-connect 102. Error free, i.e.,non-disruptive switching is realized, in accordance with an aspect ofthe invention, by employing TMS 601-1 through 601-N to each generate aso-called composite audio and video signal. The composite signalincludes all the video information from a room associated with thecorresponding VTS port and the audio mixes generated by bridge 103 to betransmitted to all of the rooms and partial audio sums to be transmittedto other MLCTs. Cross connect 102 time slot assignments of the signalsgenerated at the outputs of TMS 601-1 through 601-N are shown in FIG. 9.Accordingly, shown in FIG. 9 are the composite signals combined incross-connect 102 for each of VTS ports 500-1 through 500-N. Thus, forVTS port 500-1 the audio mix to VTS port 1 is in fixed time slots 32, 33and 34 as supplied from bridge output circuit 1005, the audio mix toVITS port 500-2 is in fixed time slots 36, 37 and 38, the audio mix toVTS port 500-3 is inserted into fixed time slots 40, 41 and 42, theaudio mix to VTS port 500-9 is in fixed time-slots 44, 45 and 46, theaudio mix to VTS port 500-5 is in fixed time slots 48, 49 and, 50 andfinally, the audio mix to VTS port 500-6 is in time slots 52, 53 and 54.The video from VTS port 500-1 is combined into time slots 64 through156. The audio from VTS port 500-1 which is in time slots 0, 1 and 2 issupplied to bridge 103 via 110-1 to be appropriately mixed with theaudio from the other VTS ports and, then, supplied via 109-1 from bridge103 to TMS 601-1 in the appropriate fixed time slots in the compositesignals associated with VTS port 500-1. The composite signals associatedwith VTS ports 500-2 through 500-6 are similarly obtained as shown inFIG. 9. Consequently, when the video from anyone of the VTS ports and,hence, the associated conference room is switched via NXN switch 602 toanyone of the other conference rooms, the proper audio mix for thespecific conference rooms is present and obtained by the MLCT selectingthe audio mix in the fixed time slots assigned to the associatedconference rooms. The functions of TMS 600 and switch 602 may berealized by employing dual multiplexers of a type described in thearticle entitled, "Digital Access and Cross-Connect System-SystemArchitecture" noted above.

FIG. 7 shows in simplified block diagram form details of timemultiplexed switches 601 of cross-connect 102 (FIG. 6). Accordingly,shown are (R+1)×1 data selector 701, TMS control memory 702 and timeslot counter 703. Selector 701 is controlled by control codes stored incontrol memory 702. The control codes are written into memory 702 frommicroprocessor system 101 and include the time slot switchinginformation for the time multiplexed switch for an assigned VTS port.Once the control information is written into control memory 702 nofurther interaction with microprocessor system 101 is required until anew data link selection is made. Control memory 702 is addressed by timeslot counter 703 which generates a sequentially incrementing modulo-256counter signal synchronized to the time slot occurrences of thecross-connect data format as shown in FIG. 3.

FIG. 8 shows in simplified block diagram form details of NXN switch 602of cross-connect 102 FIG. 6. Switch 602 includes N×1 data selectors801-1 through 801-N which are controlled by control registers 802-1through 802-N, respectively. Each of outputs 108-1 through 108-N of dataselectors 801-1 through 801-N, respectively, is connected to anassociated one of VTS ports 500-1 through 500-N (FIG. 5). Outputs 603-1through 603-N from time multiplexed switches 601-1 through 601-N (FIG.6), respectively, are supplied to respective inputs of selectors 801-1through 801-N. Switching commands are supplied from microprocessorsystem 101 via control lines 605-1 through 605-N to control registers802-1 through 802-N, respectively. In response to the switching commandseach of selectors 801-1 through 801-N selects an appropriate one to itsinputs and provides it to its associated VTS port in transmissionfacility interface 104. Consequently, any one of the composite signalssupplied from time multiplexed switches 601 can be selected fortransmission to any one of the VTS ports 500. The input selection iscontrolled to occur at the boundary between the cross-connect dataformat frame in order to prevent any mixing of information fromdifferent inputs within a given frame.

In summary, the operation of cross-connect 102 in realizingnondisruptive audio and rapid video switching, in accordance with anaspect of the invention, is such that the first 3 words of DS-1 No. 1 ofall N incoming VTS signals are identified by and formatted intopredetermined time slots by digroups circuits 501 and sent via TMS 601-1through 601-N to bridge 103 for processing the audio and controlinformation and for performing sub-byte bit manipulation ofvideo-related signaling bits located in the first word. All other wordscontaining video information are formatted into a composite video signaland provided to NXN switch 602 for the video switching function of theMLCT along with processed audio, control, and video related informationare supplied by each of data links 109-1 through 109-N from bridge 103.After any selection is made by NXN switch 602, it is up to theparticular one of digroup circuits 501 (FIG. 5) to choose the necessarytimeslots to form the corresponding outgoing VTS signal.

This implies, as will be seen in the following, that the processed audioand control signals generated in bridge 103 must appear on all datalinks 603 presented to NXN switch 602 in order to affect only videoinformation when the switch state is changed. The time-slot assignmenthas been chosen, along with other system features, so that after aconference configuration has been decided and set-up, thetime-space-time switching configuration is static except for NXN switch602. NXN switch 602 is the only switching element whose configuration isdynamic, i.e., during video switches. The other switching elementsremain constant until a new conference configuration is required, i.e.,addition or deletion of a conference room or MLCT. This type ofarrangement allows, in accordance with an aspect of the invention, veryefficient and rapid video selection operations. In fact, not using theabove method implies remapping one time-slot at a time of all theswitching elements to perform a video switch thereby resulting inserious video selection speed degradations.

Table 1 shows the relative time-slot assignment of all informationgroups as they appear at the output of TMS 601-1 through 601-N. Entriesof the form "vnwn" and "vnWn" represents a group of 3 timeslots each.The "Vn" entries represent a group of 21+(24×R) time-slots toaccommodate the video information from each VTS port. The table issubdivided into two sections each pertaining to where the information isrouted. Bridge 103, for example, has access in space and time to allaudio and control inputs to the MLCT. Conversely, bridge 103 provides Ncopies of the processed audio and control information mixed with videoportions pertinent to each VTS port. The transmit sections of digroupcircuits (DC) 500 in the transmission facility interface 104 (FIG. 4)choose the time-slots under the "VTS Output Contribution" to constructthe word pattern in the R outgoing T1 lines. Once this selection isperformed, no other interaction is done with the DC TTSI. When a "space"switch is performed by NXN switch 602 (FIG. 6) it represents a verticalmovement of the "VTS Output Contribution" section of the time-slotassignment table which affects only video information.

                                      TABLE 1                                     __________________________________________________________________________    Cross-Connect Time-Slot Assignment                                            Time-Slot Assignment                                                          VTS Input Contribution  Bridge 103 Contribution                               To Bridge 103      VTS Output Contribution                                    __________________________________________________________________________    L 1 v1w1                                                                              --  --  --  v1  v1W1                                                                             v1W2                                                                              V1Wn                                                                              v1WN                                       I 2 --  v2W2                                                                              --  --  V2  v2W1                                                                             v2W2                                                                              v2Wn                                                                              v2WN                                       N n --  --  vnwn                                                                              --  Vn  vnW1                                                                             vnW2                                                                              vnWn                                                                              vnWN                                       K N --  --  --  vNwN                                                                              VN  vNW1                                                                             vNW2                                                                              vNWn                                                                              vNWN                                       __________________________________________________________________________     LEGENDA?                                                                      vn Video in word 1 of DS1 No. 1 from VTS Input Port n                         Vn Video from VTS Input Port n                                                wn Control and Audio from VTS Input Port n                                    Wn Control and Bridged Audio to VTS Output Port n                             -- Unassigned TimeSlot Group                                             

Bridge

FIG. 10 shows in simplified block diagram form details of MLCT bridge103. Bridge 103 performs an access formatting function to cross-connect102, an audio processing function and a conference control function. Tothis end, bridge 103 includes bridge input circuit (BIC) 1001, audiodecryptor (DEC) 1002, audio bridge 1003, audio encryptor (ENC) 1004,bridge output (BOC) circuit 1005 and control processor (CP) 1006 whichare interconnected by bus 1007. Details of the components of bridge 103and their operation are shown in FIGS. 11 through 46 and describedbelow.

Briefly, outputs from time multiplexed switches 601-1 through 601-N(FIG. 6) in cross-connect 102 are supplied via 110-1 through 110-N tobridge input circuit (BIC) 1001. BIC 1001 is arranged under control ofprocessor 1006 to accept the audio time slots associated with the VTSports. These time slots include words 1, 2 and 3 as shown in the VTSdata format of FIG. 3. It separates the video signaling bits FD, X, Xfrom the control bit C and the remaining audio bits for each of VTSports 500-1 through 500-N. The video signaling bits associated with VTSports 500-1 through 500-N are supplied via circuit paths 1008-1 through1008-N, respectively, directly to bridge output circuit (BOC) 1005. Thisis necessary to keep the current frame video signaling bits with thecurrent frame video information bits. As will be apparent from thediscussion below, there is some delay in the audio because of theprocessing. The audio and control bits associated with each VTS port aresupplied to audio decryptor 1002 via 1011. Audio decryptor (DEC) 1002accesses BIC 1001 via 1012 for this purpose. DEC 1002 under control ofprocessor 1006 separates the control bit C from the remaining audioassociated with each VTS port and supplies them via 1009-1 through1009-N to control processor 1006. DEC 1002 also decrypts, if necessary,the remaining audio for all the VTS ports and supplies it via 1001 toaudio bridge 1003. The decryption process is performed using anappropriate decryption key supplied from microprocessor system 101during the conference setup. The decryption process may be bypassed ifthe conference is not using encryption of the information. Audio bridge1003 accesses an output port in DEC 1002 via 1014. Audio bridge 1003generates the appropriate audio mixes for each VTS port. The audio mixfor each of the VTS ports includes the audio from all other ports exceptitself. For example, the audio mix for VTS port 1 includes only theaudio from VTS ports 2 through N. The method with which audio bridgesums the audio varies according to the number of data presently employedin a conference and whether the audio sum is destined to a conferenceroom or to another MLCT. The audio sums are then supplied via 1015 and1016 to audio encryptor (ENC) 1004. Also supplied to ENC 1004 via 1010-1through 1010-N are modified control bits associated with each of VTSports 1 through N, respectively. ENC 1004 encrypts, if necessary, theaudio sums in accordance with an encryption key supplied frommicroprocessor system 101 during setting up of the conference. Again, ifno encryption is being used the encryption process in ENC 1004 isbypassed. Additionally, ENC 1004 combines the modified control bitsgenerator by processor 1006 with the appropriate audio sums and suppliesthem via 1017 and 1018 to bridge output circuit (BOC) 1005. Bridgeoutput circuit 1005 combines the processed audio from ENC 1004 with thevideo signaling bits from BIC 1001 and formats a cross-connect 102compatible signal for each VTS port with time-slot assignmentsconsistent with those of Table 3.

Control processor 1006 in addition to control of bridge 103 performs allcommunications via the auxiliary C bit data channel to the conferencerooms and other MLCTs in the conference for realizing necessarycommunications and video operations within the MLCT. Additionally,processor 1006 switches messages to be transmitted from room to room.Within bridge 103, processor 1006 communicates via parallel bus 1007.

Bridge Input Circuit (BIC)

FIG. 11 shows in simplified block diagram form details of bridge inputcircuit (BIC) 1001. As indicated above, BIC 1001 separates the videosignaling bits FD, X, X from the audio information for each VTS port. Tothis end, the outputs from time multiplexed switches 601-1 through 601-N(FIG. 6) are supplied via 110-1 through 110-N, respectively, to inputsof NX1 data selector 1101. Selector 1101 is controlled by control codesstored in control memory 1102 to perform the space selection function ofthe audio signals from the VTS ports shown in cross-connect 102 timeslot assignments in FIG. 9. The control codes are written into memory1102 from control processor 1006 (FIG. 10) and include the time slotswitching information for the supplied data links from the timemultiplexed switches 601-1 through 601-N. The output from selector 1101is supplied to the data input of video latch 1103 and to data memory1104. Video latch 1103 includes N latch circuits which are assigned on aone-to-one basis to data links 110-1 through 110-N and are used totemporarily store the video signaling bits FD, X, X from those links. Tothis end, control memory 1102 also includes codes supplied from controlprocessor 1006 (FIG. 10) for enabling the appropriate latch circuits invideo latch 1103 to temporarily store the corresponding video signalingbits which are then supplied via 1008-1 through 1008-N to bridge outputcircuit 1005. Each code word includes 8 bits and is associted with aspecific time slot in the cross connect frame format shown in FIG. 3.Two of the bits are spares and two bits are used for parity. Theremaining four bits are so-called operational control bits and aredefined as follows:

b3: RVCSEN. Receive Video Control Slot. A logical 1 indicates that therespective time slot contains video control bits FD, X, X to be latchedin latch 1103 and transmitted to BOC 1005. A logical 0 indicates anormal time slot.

b2: RPA2. Receive Port Address (MSB).

b1: RPA1. Receive Port Address.

b0: RPA0. Receive Port Address (LSB).

Bits b0-b2 form a three bit code indicating the origin of data to bereceived by bridge 103. Codes 0-5 select the respective VTS ports. Codes6 or 7 will not select any port and BIC 1001 assumes that the associatetime slot is idle. Data memory 1104 is a dual port memory written withthe remaining audio and control information from selector 1101. Memory1104 is accessed by decryptor 1002 via 1012 further to process thestored audio and control information. Data memory 1104 includes a RAMhaving duplicate sets of memory locations. A first set of the RAMlocations is written with incoming data from selector 1101 while thesecond set of RAM locations is read by decryptor 1002 during a currentframe. During a next subsequent frame the first set is read while thesecond set is written. Both control memory 1102 and data memory 1104 aresequentially addressed by time-slot counter 1105 which includes amodulo-256 binary counter synchronized to the internal frame time-slotboundaries. Once a code pattern is written into control memory 1102 noother interaction is required unless a VTS port and, hence, a conferenceroom is to be added to or deleted from the conference.

Encryption/Decryption Process

Audio decryptor (DEC) 1002, audio bridge 1003 and audio encryptor (ENC)1004 form, in accordance with an aspect of the invention, a programmableaudio mixing arrangement including common control decryption andencryption. Such an arrangement is advantageously utilized incontrollably adding rooms to and/or deleting rooms from an ongoingconference.

Private communication is realized by separately encrypting both theaudio and video information. The audio information from the rooms mustbe decrypted in the MLCT to perform the audio mixing in bridge 1003.Then, the mixed audio is encrypted on a common control basis in ENC1004.

For clarity and simplicity of description the general encryption processshall be discussed first. As described below encryption of the audio isobtained by using the Data Encryption Standard (DES) promulgated by theNational Bureau of Standards.

The audio information is encrypted by adding each audio bit modulo-2 toa synchronous stream of cipher bits derived from the DES algorithm. Thedata input to be encrypted is derived from a 64-bit plain text counterhaving arbitrary contents which are known to both ENC 1004 and DEC 1002.The encryption process in the DES algorithm uses an appropriate 56 bitsecret key in conjunction with the contents of the plain text counter toform the cipher bit stream which the audio is added modulo-2 bit-by-bitthereby forming the encrypted audio (A-bits). The encryption process maybe easily bypassed. The audio information is sampled in a conferenceroom by picture processor 123 (FIG. 1) and encoded with standard u-255companding. Therefore, each 125 mu-sec frame includes two adjacentsamples of the audio, as shown in Table 2 below.

Decryptor 1002 employs a common control routine to decrypt a pluralityof simultaneously received, randomly phased encrypted audio signals byadding modulo-2 to each received signal a cipher bit stream synchronizedwith and identical to the one used for encrypting the signal. This isrealized by use of the 56 bit encryption key and knowledge of the 64 bitplain text counter contents at the time the cipher stream was generated.The plain text counter state is sent over the encryption deciphermentbits (D-bits). Also included in the D-bits are a framing pattern and aparity bit which provides EVEN parity over the unencrypted audiosamples. This parity bit is used by decryptor 1002 to verify thatdecryption has been performed correctly. If the decryption is incorrectbecause of a data link failure, the decryptor will cause that link'saudio to be muted. This insures that the audio performance of theconference is not degraded by the link failure.

FIG. 12 shows all the data associated with audio encryptor 1004 outputsignal for a VTS link in addition to the audio bits (A-bits) shown inTable 2. The relationship of the A-bits to the mu-255 companding law isshown in Table 2. Note that the A-bits do not represent the complementedvalues of the mu-255 codes as in a normal DS-1 format.

                  TABLE 2                                                         ______________________________________                                        VTS Data Format: Audio Group                                                  A-bit      Mu-law  Description                                                ______________________________________                                        A1     A9      M0      Mantissa Least Significant Bit                         A2     A10     M1                                                             A3     A11     M2                                                             A4     A12     M3      Mantissa Most Significant Bit                          A5     A13     L0      Cord Least Significant Bit                             A6     A14     L1                                                             A7     A15     L2      Cord Most Significant Bit                              A8     A16     S       Sign                                                   ______________________________________                                    

FIG. 12 shows the D-bit sequence. The D-bits are decipherment controlbits which are used for synchronization of the audio encryption anddecryption circuitry. The D-bits are an 8 Kb/sec data stream organizedinto a 128 bit superframe. The D-bits are organized into an odd-numberedgroup and an even numbered group. The superframe also includes 32 mostsignificant bits (MSB) of the counter contents, namely L1-L32 which areused during the next superframe interval, and 32 check bits, namely,R1-R32. The 32 plain text counter contents bits L1-L32 are subdividedinto 8 blocks of 4 bits. Appended to each block of 4 bits is arespective block of 4 error control check bits labeled R1 through R32.The L and R bits implement eight blocks of (8, 4) extended Hamming codewords. The even-numbered D-bits contain a fixed sequence of framing bitslabeled S1 through S6 and FA0 through FA7 and a set of audio paritybits. The value of the audio parity bits is such that in the frame thatthey occur, even parity is obtained over the on-the-clear audio samplerepresented by A9 through A16. This information is shown in TABLE 3below.

                  TABLE 3                                                         ______________________________________                                         VTS Data Format: Audio Decipherment Group                                    ______________________________________                                        Odd Numbered D-bits                                                           ______________________________________                                        n   Counter Contents  Check Bits                                              ______________________________________                                        0   L1      L2     L3    L4   R1    R2    R3   R4                             1   L5      L6     L7    L8   R5    R6    R7   R8                             2   L9      L10    L11   L12  R9    R10   R11  R12                            3   L13     L14    L15   L16  R13   R14   R15  R16                            4   L17     L18    L19   L20  R17   R18   R19  R20                            5   L21     L22    L23   L24  R21   R22   R23  R24                            6   L25     L26    L27   L28  R25   R26   R27  R28                            7   L29     L30    L31   L32  R29   R30   R31  R32                            ______________________________________                                        R4n+1 = L4n+1 ⊕ L4n+2 ⊕ L4n+3                                         R4n+2 = L4n+2 ⊕ L4n+3 ⊕ L4n+4                                         R4n+3 = L4n+1 ⊕ L4n+2 ⊕ L4n+4                                         R4n+4 = L4n+1 ⊕ L4n+3 ⊕ L4n+4                                         Even Numbered D-bits                                                          ______________________________________                                        n    S1      S2     S3    S4   S5   S6    P    FAn                            ______________________________________                                        0    1       0      0     1    1    1     P    0                              1    1       0      0     1    1    1     P    0                              2    1       0      0     1    1    1     P    0                              3    1       0      0     1    1    1     P    0                              4    1       0      0     1    1    1     P    1                              5    1       0      0     1    1    1     P    1                              6    1       0      0     1    1    1     P    1                              7    1       0      0     1    1    1     P    1                              ______________________________________                                         The symbol ⊕ indicates an "Exclusive OR" function.                   

For encryption, the 64 bit plain text counter used to obtain the cipherstream is subdivided into two sections, one including L1-L32representing the most significant bits and another 11-132 representingthe 32 least significant bits (LSB). At the end of a D-bit superframe,the 32 most significant bits presently sent via the D-bits are utilizedas a portion of the plain text during the next superframe and,therefore, are stored for that purpose. The L1-L32 section of the plaintext counter is incremented by 1 and the resulting plain text countercontents are used for encryption. The 11-132 bit section is reset to 0,and every 4 DS-1 frames (125 mu sec) the most 5 significant bits 28through 32 are incremented and used to perform a plain text counterencryption. One encryption is required every 4 DS-1 frames, because ineach frame 16 bits of the cipher stream are used to perform thebit-by-bit modulo-2 sum of the audio and cipher stream. This is turnedto account, in accordance with an aspect of the invention, in obtainingthe common control encryption and decryption processes described below.The decryptor is able to derive the same cipher stream that was used forencryption because it has obtained the most significant 32 bits (L1-L32)in use by the encryptor during the previous D-bit superframe. Theprocess repeats after 32 encryptions or 128 D-bits have beentransmitted. A timing diagram of the encryption process is shown in FIG.13. The plain text counter section containing the most significant 32bits L1-L32 is initialized to the preset values as shown in FIG. 13. Theidentifying code is an arbitrary number chosen by the user.

In the decryption process, the plain text data input to the DES moduleis derived from the received 32 most significant bits L1-L32 used toderive the cipher bit stream during the encryption process. The leastsignificant 32 bit portions of the DES plain text data input, namely,bits 11-132, comprise a plain text counter identical to that used in theencryption process. A framer function locates the phase of the 128 bitsuperframe boundaries and extracts the information shown in TABLE 3above, and supplies the audio parity bit to a parity checker during theframe in which the D-bits occur. The parity check generates a ciphererror indication. In turn, the cipher error ORed with an out of D-bitsuperframe indication generates a mute signal which is used to inhibitany erroneous audio information from impairing the conference.

As in the encryption process, the decryption process may readily bebypassed by generating a cipher stream of all 0's.

Audio Decryptor (DEC)

Details of audio decryptor (DEC) 1002 of FIG. 10 are shown in simplifiedblock diagram form in FIG. 14. DEC 1002 provides, in accordance with anaspect of the invention, a common control arrangement for decrypting amultiplicity of VTS audio channels. Accordingly, shown aremicrocontroller 1401 and associated program memory 1402, BIC 1001 datainput port 1403, BIC 1001 data read control port 1404, scratchpad RAMmemory 1405, serial output port 1406, DES data encryption standardmodule 1407, control channel output port 1408 and control processor businterface 1409. Microcontroller 1401 is interconnected with units 1403through 1409 via bus 1410. Additionally, microcontroller 1401 receives areset signal from interface 1409 and code instructions from programmemory 1402 in response to address signals supplied thereto. Programmemory 1402 is also connected to interface 1409. In this example,microcontroller 1401 is a Signetics 8×305 and DES module 1407 includes aFairchild 9414-1,-2,-3,-4 DES chip set which implements the NationalBureau of Standards (NBS) Data Encryption Standard (DES).

DEC 1002 accesses the control channel audio, and audio decipherment bits(D-bits) stored in BIC 1001 data memory 1104 (FIG. 11) by supplying anappropriate address via BIC data read control port 1404 over 1012 and byreading the contents of BIC data memory 1104 via BIC data input port1403. DEC 1002 under control of microcontroller 1401 separates thecontrol C-bits from the stored information and stores them in controlchannel output port 1208 for further processing by control processor1006. The audio and audio decipherment bits are operated on to generatedeciphered audio for all the VTS ports. The deciphered audio is suppliedvia serial output port 1406 to audio bridge 1003 for further processing.Control processor 1006 accesses DEC 1002 via control processor businterface 1409. Interface 1409 provides read-write access tomicrocontroller program memory 1402 for loading the decryptor programinto memory 1402, for verifying that the program is stored properly andto halt and restart microprocessor 1401 as appropriate. Oncemicrocontroller 1401 is in the run mode, control processor 1006communicates to microcontroller 1401 via bus 1410 for communicatingfault conditions associated with the audio decryption operators. Thesefault conditions are used, in accordance with an aspect of theinvention, to determine whether a conference room can be added to ordeleted from an ongoing conference without disruption of the conference.Additionally, control processor 1006 (FIG. 39) supplies the DES key andthe encryption modes, namely, encrypted/clear via interface 1409 to DEC1002.

Moreover, DEC 1002 realizes common control decryption of the incomingaudio signals under program control. Consequently, rooms may bedynamically added to or deleted from and ongoing conference withoutdisrupting it, in accordance with an aspect of the invention.

Operation of audio decryptor (DEC) 1002 is controlled by microcontroller1401 under control of programs stored in program memory 1402.

FIG. 15 is a flowchart of a program routine stored in program memory1402 to effect, in accordance with an aspect of the invention, thecommon control decryption of the simultaneously received, randomlyphased incoming VTS audio signals. Accordingly, the program is enteredvia oval 1501. Thereafter, operational block 1502 causes initializationof system variables to known states. For example, the ENC mode is set tobypass encryption if transmission is in the clear and AMUTST is set tomuted until it is determined that the rooms are being decryptedproperly, among others.

Conditional branch point 1503 tests to determine the beginning of theDS-1 frame, i.e., DS-1 frame sync, of the VTS signal. If the test resultis true, operational block 1504 sets counter J to 1. is detected.Thereafter, operational block 1504 sets counter J to 1.

Operational block 1505 provides time slot address information for port Jto BIC data read control port 1404. Operational block 1506 calls thePADEC subroutine to perform audio decryption and to extract the controlchannel information for port J.

The PADEC subroutine is shown in FIGS. 16, 17, 18 and 19 and describedbelow.

Operational block 1507 causes the audio information for port J to bestored in an associated portion of scratchpad memory 1405.

Operational block 1508 causes the control channel information i.e., bitCJ, to be supplied to control processor 1006 via an associated one ofdata links 1009-1 through 1009-N.

Operational block 1509 causes the mute status supplied by subroutinePADEC to be stored in an AMUTST location associated with port J inscratchpad memory 1405.

Operation block 1510 sets counter J equal to J+1.

Conditional branch point 1511 tests to determine if J is greater than N.If the test result is false, control is returned to operational block1507 and steps 1505 through 1511 are iterated until the test result instep 1511 is true, i.e., yes.

Conditional branch point 1512 tests to determine whether the mute statusknown by the decryptor is the same as the one known by the controlprocessor 1006. If the test result is false, an interrupt is sent tocontrol processor 1006 via operational block 1513. If the test result instep 1512 is true, no interrupt is sent to control processor 1006.Operational block 1514 causes the transferral of audio information fromthe scratchpad memory 1405 to audio bridge 1003 via data link 1013.Thereafter, control is returned to conditional branch point 1503. If thetest result of conditional branch point 1503 is false, operational block1515 calls subroutine CENCHR which performs a plain text counterencryption if requested and checks for requests from control processor1006. The CENCHR subroutine is shown in FIGS. 20 and 21 and describedbelow.

FIGS. 16, 17, 18, and 19 connected as shown illustrate a flowchart ofthe PADEC subroutine employed in operational block 1506 of the decryptorprogram routine of FIG. 15. It is important to note that the D-bitsequences of the incoming audio signals are randomly aligned.Consequently, the entire decryption process cannot simply be performed Ntimes and different operations must be performed on each VTS audiosignal. These operations are achieved, in accordance with an aspect ofthe invention, by employing the PADEC subroutine. Accordingly, the PADECsubroutine is entered via oval 1601.

Operational block 1602 causes the BIC data input port 1403 (FIG. 14) toread the data from bridge input circuit 1001 associated with the audio,control channel and audio decipherment information. The data is suppliedvia data link 1011 in accordance with the address supplied to bridgeinput circuit 1001 via operational block 1505 (FIG. 15).

Operational blocks 1603, 1604 and 1605 cause segregation of the audiodata, control channel data and decipherment information, respectively.Operational block 1606 increments a D-bit counter, i.e., a statecounter, for VTS channel J. The state counter is a modulo-128 counter,i.e., counts from 0 to 127 and repeats. The D-bit counter is used on aper channel basis to keep track of operations in performing thedecryption.

Operational block 1607 implements a 128 way branch based on the state ofthe least seven significant bits of the D-bit counter.

Operational block 1608 implements a set of operations based on each ofthe 128 states of the D-bit state counter on a per channel basis asshown in Table 4 below. The information provided in Table 4 issubdivided into 6 columns as shown.

Column "K" is the D-bit number as described by the state of the D-bitcounter in memory 1405 (FIG. 14).

Column "D-bit K Processing" describes the operation that should beperformed on each of the different D-bits as shown in FIG. 12.

Column "Received Counter Contents Bookkeeping" assembles the countercontents to be utilized as plain text into the DES module 1407 (FIG.14).

Column "Counter Encryption Request" requests encryption of the plaintext counter contents.

Column "Cipher Byte Selection to Decrypt Audio" determines which ciphertext block is to be used to perform audio decryption as described below.

Column "Flow Chart Node" directs the operation to one of flowchart nodesS, LR, or P.

In TABLE 4 the following are used:

LRTMP is a temporary register in memory 1405;

NXTPT is a next plain text register in memory 1405;

HAM(LRTMP) is the code correction of L bits as per TABLE 3;

PRSPT is a present plain text register in memory 1405;

ENCRQ is an encryption request flag including cipher text block [CT]1 or[CT]0; and

CTADDR is a cipher text address in memory 1405.

It is noted that during a present four (4) frame block of audioinformation, e.g., K=1 through 4, encryption of the plain text countercontents and the resulting cipher text generated by DES module 1407 tobe stored, for example, in cipher text (CT) block 1, will be used todecrypt the next subsequent 4 frame block, e.g., K=5 through 8, of audioinformation. This is realizable because the cipher text is 64 bits andthe audio sample is only 16 bits. Thus, the cipher text is used withfour audio samples. Since only one cipher text word is needed for everyfour audio samples per channel the DES module is further freed-up foruse with the other channels. During the next 4 frame block of audioinformation encryption of the plain text counter contents and resultingcipher text stored in [CT] block 0 will be used for decrypting the nextblock of audio information. That is to say, the cipher text blocksalternate for each 4 frame block of audio information. Moreover, sincesingle DES module 1407 (FIG. 14) needs to be used on a per channel basisonly once every 4 frames, and since the DES module performs theprescribed operation in a fraction of an audio sample period, it can beused to perform similar operations for the other VTS channels.Additionally, it is noted that DES module 1407 takes less time than oneframe interval to perform the encryption operation on the plain textdata. Thus, by employing common control techniques, in accordance withan aspect of the invention, the operations shown in TABLE 4 below areconcurrently being performed for up to N randomly phased signals from NVTS channels.

                                      TABLE 4                                     __________________________________________________________________________                 Received                                                                      Counter   Counter                                                                             Cipher Byte                                                                             Flow                                      D-bitK    Contents  Encryption                                                                          Selection to                                                                            Chart                                  K  Processing                                                                              Bookkeeping                                                                             Request                                                                             Decrypt Audio                                                                           Node                                   __________________________________________________________________________    001                                                                              RTMPO=D=L1          ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (1,[Ct]1)                                              002                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      003                                                                              LRTMP1=D=L2               CTADDR=*[Ct2]0                                                                          LR                                     004                                                                              FRAMING=D+1               CTADDR=*[Ct3]0                                                                          S                                      005                                                                              LRTMP2=D=L3         ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                            (2,[Ct]0)                                              006                                                                              FRAMING=D+1               CTADDR=*[Ct1]1                                                                          S                                      007                                                                              LRTMP3=D=L4               CTADDR=*[Ct2]1                                                                          LR                                     008                                                                              FRAMING=D+0               CTADDR=*[Ct3]1                                                                          S                                      009                                                                              LRTMP4=D=R1         ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (3,[Ct]1)                                              010                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      011                                                                              LRTMP5=D=R2               CTADDR=*[Ct2]0                                                                          LR                                     012                                                                              FRAMING=D+0               CTADDR=*[Ct3]0                                                                          S                                      013                                                                              LRTMP6=D=R3                                                                             NXTPT0 =  ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                  HAM(LRTMP)                                                                              (4,[Ct]0)                                              014                                                                              PARITY=D                  CTADDR=*[Ct1]1                                                                          P                                      015                          CTADDR=*[Ct2]1                                                                          LR                                     016                                                                              FRAMING=D+1               CTADDR=*[Ct3]1                                                                          S                                      017                                                                              LRTMPO=D=L5         ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (5,[Ct]1)                                              018                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      019                                                                              LRTMP1=D=L6               CTADDR=*[Ct2]0                                                                          LR                                     020                                                                              FRAMING=D+1               CTADDR=*[Ct3]0                                                                          S                                      021                                                                              LRTMP2=D=L7         ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                            (6,[Ct]0)                                              022                                                                              FRAMING=D+1               CTADDR=*[Ct1]1                                                                          S                                      023                                                                              LRTMP3=D=L8               CTADDR=*[Ct2]1                                                                          LR                                     024                                                                              FRAMING=D+0               CTADDR=*[Ct3]1                                                                          S                                      025                                                                              LRTMP4=D=R5         ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (7,[Ct]1)                                              026                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      027                                                                              LRTMP5=D=R6               CTADDR=*[Ct2]0                                                                          LR                                     028                                                                              FRAMING=D+0               CTADDR=*[Ct3]0                                                                          S                                      029                                                                              LRTMP6=D=R7                                                                             NXTPT1 =  ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                  HAM (LRTMP)                                                                             (8,[Ct]0)                                              030                                                                              PARITY=D                  CTADDR=*[Ct1]1                                                                          P                                      031                          CTADDR=*[Ct2]1                                                                          LR                                     032                                                                              FRAMING=D+1               CTADDR=*[Ct3]1                                                                          S                                      033                                                                              LRTMPO=D=L9         ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (9,[Ct]1)                                              034                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      035                                                                              LRTMP1=D=L10              CTADDR=*[Ct2]0                                                                          LR                                     036                                                                              FRAMING=D+1               CTADDR=*[Ct3]0                                                                          S                                      037                                                                              LRTMP2=D=L11        ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                            (10,[Ct]0)                                             038                                                                              FRAMING=D+1               CTADDR=*[Ct1]1                                                                          S                                      039                                                                              LRTMP3=D=L12              CTADDR=*[Ct2]1                                                                          LR                                     040                                                                              FRAMING=D+0               CTADDR=*[Ct3]1                                                                          S                                      041                                                                              LRTMP4=D=R9         ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (11,[Ct]1)                                             042                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      043                                                                              LRTMP5=D=R10              CTADDR=*[Ct2]0                                                                          LR                                     044                                                                              FRAMING=D+0               CTADDR=*[Ct3]0                                                                          S                                      045                                                                              LRTMP6=D=R11                                                                            NXTPT2 =  ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                  HAM(LRTMP)                                                                              (12,[Ct]0)                                             046                                                                              PARITY=D                  CTADDR=*[Ct1]1                                                                          P                                      047                          CTADDR=*[Ct2]1                                                                          LR                                     048                                                                              FRAMING=D+1               CTADDR=*[Ct3]1                                                                          S                                      049                                                                              LRTMPO=D=L13        ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (13,[Ct]1)                                             050                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      051                                                                              LRTMP1=D=L14              CTADDR=*[Ct2]0                                                                          LR                                     052                                                                              FRAMING=D+1               CTADDR=*[Ct3]0                                                                          S                                      053                                                                              LRTMP2=D=L15        ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                            (14,[Ct]0)                                             054                                                                              FRAMING=D+1               CTADDR=*[Ct1]1                                                                          S                                      055                                                                              LRTMP3=D=L16              CTADDR=*[Ct2]1                                                                          LR                                     056                                                                              FRAMING=D+0               CTADDR=*[Ct3]1                                                                          S                                      057                                                                              LRTMP4=D=R13        ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (15,[Ct]1)                                             058                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      059                                                                              LRTMP5=D=R14              CTADDR=*[Ct2]0                                                                          LR                                     060                                                                              FRAMING=D+0               CTADDR=*[Ct3]0                                                                          S                                      061                                                                              LRTMP6=D=R15                                                                            NXTPT3 =  ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                  HAM(LRTMP)                                                                              (16,[Ct]0)                                             062                                                                              PARITY=D                  CTADDR=*[Ct1]1                                                                          P                                      063                          CTADDR=*[Ct1]1                                                                          LR                                     064                                                                              FRAMING=D+1               CTADDR=*[Ct3]1                                                                          S                                      065                                                                              LRTMPO=D=L17        ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (17,[Ct]1)                                             066                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      067                                                                              LRIMP1=D=L18              CTADDR=*[Ct2]0                                                                          LR                                     068                                                                              FRAMING=D+1               CTADDR=*[Ct3]0                                                                          S                                      069                                                                              LRTMP2=D=L19        ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                            (18,[Ct]0)                                             070                                                                              FRAMING=D+1               CTADDR=*[Ct1]1                                                                          S                                      071                                                                              LRTMP3=D=L20              CTADDR=*[Ct2]1                                                                          LR                                     072                                                                              FRAMING=D+0               CTADDR=*[Ct3]1                                                                          S                                      073                                                                              LRTMP4=D=R17        ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (19,[Ct]1)                                             074                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      075                                                                              LRTMP5=D=R18              CTADDR=*[Ct2]0                                                                          LR                                     076                                                                              FRAMING=D+0               CTADDR=*[Ct3]0                                                                          S                                      077                                                                              LRTMP6=D=R19                                                                            NXTPT4 =  ENCRQ CTADDR=*[ Ct0]1                                                                         LR                                                  HAM(LRTMP)                                                                              (20,[Ct]0)                                             078                                                                              PARITY=D                  CTADDR=*[Ct1]1                                                                          P                                      079                          CTADDR=*[Ct2]1                                                                          LR                                     080                                                                              FRAMING=D+1               CTADDR=*[Ct3]1                                                                          S                                      081                                                                              LRTMPO=D=L21        ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (21,[Ct]1)                                             082                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      083                                                                              LRTMP1=D=L22              CTADDR=*[Ct2]0                                                                          LR                                     084                                                                              FRAMING=D+1               CTADDR=*[Ct3]0                                                                          S                                      085                                                                              LRTMP2=D=L23        ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                            (22,[Ct]0)                                             086                                                                              FRAMING=D+1               CTADDR=*[Ct1]1                                                                          S                                      087                                                                              LRTMP3=D=L24              CTADDR=*[Ct2]1                                                                          LR                                     088                                                                              FRAMING=D+0               CTADDR=*[Ct3]1                                                                          S                                      089                                                                              LRTMP4=D=R21        ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (23,[Ct]1)                                             090                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      091                                                                              LRTMP5=D=R22              CTADDR=*[Ct2]0                                                                          LR                                     092                                                                              FRAMING=D+0               CTADDR=*[Ct3]0                                                                          S                                      093                                                                              LRTMP6=D=R23                                                                            NXTPT5 =  ENCRQ CTADDR=*[Ct0] 1                                                                         LR                                                  HAM(LRTMP)                                                                              (24,[Ct]0)                                             094                                                                              PARITY=D                  CTADDR=*[Ct1]1                                                                          P                                      095                          CTADDR=*[Ct2]1                                                                          LR                                     096                                                                              FRAMING=D+1               CTADDR=*[Ct3]1                                                                          S                                      097                                                                              LRTMPO=D=L25        ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (25,[Ct]1)                                             098                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      099                                                                              LRTMP1=D=L26              CTADDR=*[Ct2]0                                                                          LR                                     100                                                                              FRAMING=D+1               CTADDR=*[Ct3]0                                                                          S                                      101                                                                              LRTMP2=D=L27        ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                            (26,[Ct]0)                                             102                                                                              FRAMING=D+1               CTADDR=*[Ct1]1                                                                          S                                      103                                                                              LRTMP3=D=L28              CTADDR=*[Ct2]1                                                                          LR                                     104                                                                              FRAMING=D+0               CTADDR=*[Ct3]1                                                                          S                                      105                                                                              LRTMP4=D=R25        ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (27,[Ct]1)                                             106                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      107                                                                              LRTMP5=D=R26              CTADDR=*[Ct2]0                                                                          LR                                     108                                                                              FRAMING=D+0               CTADDR=*[Ct3]0                                                                          S                                      109                                                                              LRTMP6=D=R23                                                                            NXTPT6 =  ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                  HAM(LRTMP)                                                                              (28,[Ct]0)                                             110                                                                              PARITY=D                  CTADDR=*[Ct1]1                                                                          P                                      111                          CTADDR=*[Ct2]1                                                                          LR                                     112                                                                              FRAMING=D+1               CTADDR=*[Ct3]1                                                                          S                                      113                                                                              LRTMPO=D=L29        ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (29,[Ct]1)                                             114                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      115                                                                              LRTMP1=D=L30              CTADDR=*[Ct2]0                                                                          LR                                     116                                                                              FRAMING=D+1               CTADDR=*[Ct3]0                                                                          S                                      117                                                                              LRTMP2=D=L31        ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                            (30,[Ct]0)                                             118                                                                              FRAMING=D+1               CTADDR=*[Ct1]1                                                                          S                                      119                                                                              LRTMP3=D=L32              CTADDR=*[Ct2]1                                                                          LR                                     120                                                                              FRAMING=D+0               CTADDR=*[Ct3]1                                                                          S                                      121                                                                              LRTMP4=D=R29        ENCRQ CTADDR=*[Ct0]0                                                                          LR                                                            (31,[Ct]1)                                             122                                                                              FRAMING=D+0               CTADDR=*[Ct1]0                                                                          S                                      123                                                                              LRTMP5=D=R30              CTADDR=*[Ct2]0                                                                          LR                                     124                                                                              FRAMING=D+0               CTADDR=*[Ct3]0                                                                          S                                      125                                                                              LRTMP6=D=R31                                                                            NXTPT6 =  ENCRQ CTADDR=*[Ct0]1                                                                          LR                                                  HAM(LRTMP)                                                                              (0,[Ct]0)                                                           PRSPT=NXTPT                                                      126                                                                              PARITY=D                  CTADDR=*[Ct1]1                                                                          P                                      127                          CTADDR=*[Ct2]1                                                                          LR                                     128                                                                              FRAMING=D+1               CTADDR=*[Ct3]1                                                                          S                                      __________________________________________________________________________

Flow chart node S is entered as described in Table 4. Thereafter,operational block 1609 will choose the proper cipher text as shown inTable 4 to decrypt the audio information in bits A-1 through A-16 byperforming an Exclusive OR function of the chosen cipher text portionand the audio sample AJ.

Conditional branch point 1610 tests to determine whether the D-bitcounter state is in synchronism with the audio deciphering bits superframe (D-bit super frame). If the test result is true, there issynchronism and control is transferred to conditional branch point 1611.However, if the test result is false, operational block 1612 decrementsthe D-bit counter to search for D-bit superframe synchronism. Then,operational block 1613 mutes the audio by forcing bits A-1 through A-16to be logical 0's and also sets the mute flag to a logical 1, therebyindicating that the audio has been muted to control processor 1006.Conditional branch point 1611 tests to determine whether the mutecounter is equal to 0. If the test result is true, operational block1614 sets the mute flag to a logical 0 thereby indicating that the audiois not muted. If the test result in step 1611 is false, the audio bitsA-1 through A-16 are set to logical 0's and the mute flag is set to alogical 1, again indicating that the audio is muted. Control is returnedto the decryptor program routine (FIG. 15) via oval 1616.

Flow chart node LR is entered as described in Table 4. Thereafter,operational block 1617 will choose the proper cipher text as shown inTable 4 to decrypt the audio information in bits A-1 through A-16 byperforming an Exclusive OR function of the chosen cipher text portionand the audio samples AJ.

Conditional branch point 1618 tests to determine if the mute counter isset to 0. If the test result is true, operational block 1619 sets themute flag to 0 indicating that the audio is not muted. If the testresult in step 1618 is false, operational block 1620 sets audio bits A-1through A-16 to logical 0's and sets the mute flag to a 1, therebyindicating that the audio is muted. Thereafter, control is returned tothe decryptor program routine (FIG. 15) via oval 1621.

Flowchart node P is entered as described in Table 4. Thereafter,operational block 1622 decrypts audio bits A-1 through A-16 byperforming an Exclusive OR function of the chosen cipher text portionand the audio sample AJ.

Operational block 1623 computes the modulo-2 sum over the decrypted bitsA-9 through A-16 and the associated value of the D-bit.

Conditional branch point 1624 tests to determine whether the modulo-2sum resulted in an error defined as odd parity over audio bits A-9through A-16. If the test result is false, control is transferred toconditional branch point 1625. However, if the test result is true,operational block 1626 sets the mute counter to 15 to ensure at least 15error free audio samples, corresponding to when the D-bit is a paritybit (a total of 240 audio samples), are received before the audio isunmuted. Operational block 1627 sets the audio bits A-1 through A-16 tological 0's and sets the mute flag to 1, thereby indicating that theaudio is muted.

Conditional branch point 1625 tests whether the mute counter equals 0.If the test result is true, operational block 1628 sets the mute flag to0, thereby indicating that the audio is unmuted. If the test result instep 1625 is false, operational block 1629 decrements the mute counter.Thereafter, operational block 1630 sets audio bits A-1 through A-16 tological 0 and sets the mute flag to 1, thereby indicating that the audiois muted. Then, control is returned to the decryptor program routine(FIG. 15) via oval 1631.

Thus, the PADEC routine controls operation of DEC 1002 to effect acommon time shared function which is responsive to the deciphermentcontrol bits (D-bits) from each of a plurality of received audio signalsfor obtaining plain text data corresponding to each of the receivedaudio signals and for generating corresponding encryption requests asset out in Table 4 above. Additionally, the PADEC routine controls DEC1002 to controllably utilize, again on a common time shared basis, thecipher text data generated by the CENCHR routine described below fordecrypting the received audio samples. As described above, the PADECroutine employs a plurality of process state counters, i.e., DBCNTRJstep 1606, for directing operations, in accordance with Table 4, foreach of the received signal channels on a time shared basis.

It is noted that the common control arrangement of the invention doesnot merely repeat the same function N times for the N audio signals butcan perform any one of a multiplicity of operations for each of the Naudio signals as directed by the process state counter, i.e., D-bitcounter associated with the particular audio signal. Thus, thedecryptor, in accordance with the invention, decrypts simultaneouslyreceived, randomly phased audio signals. In this example, 128 operationsare directed by each of the D-bit counters and each of the N channelsmay require a different one of those operations.

It is further noted, that although the invention is employed in thisexample to decrypt audio signals, it is equally applicable to any othertype of data signal.

FIGS. 20 and 21 connected as shown form a flowchart of the CENCHRsubroutine called in operational blocks 1515 and 1517 of the decryptorprogram routine shown in FIG. 15. The CENCHR subroutine is thecontroller for performing the plain text encryption for each of the VTSaudio signals. Accordingly, the CENCHR subroutine is entered via oval2001. Thereafter, operational block 2002 sets counter J to 1.

Conditional branch point 2003 tests whether data link J contains anencryption request according to Table 4 of operational block 1608 of thePADEC subroutine shown in FIG. 16. If the test is false, operationalblock 2004 sets counter J equal J+1.

Conditional branch point 2005 tests whether J>N. If the test result isfalse, control is returned to conditional branch point 2003. If the testresult in step 2005 is true, conditional branch point 2006 tests whetherthe control processor 1006 requested a command to be executed by theaudio decryptor. If the test result in step 2006 is false, control isreturned to the decryptor program routine of FIG. 15 via oval 2007.However, if the test result in step 2006 is true, operational block 2008reads the control processor command and causes one of four operations tobe effected depending on the command which is read. The operations areread encryption mode, set encryption mode, read memory 1405, and writememory 1405, which are effected by operational blocks 2009, 2010, 2011and 2012, respectively.

Operational block 2009 transfers the encryption mode in the ENCMODEregister to register CPIO located in control processor bus interface1409 of FIG. 14.

Operational block 2010 sets the register ENCMODE to the value inregister CPIO located in control processor bus interface 1409.

Conditional branch point 2013 tests whether the decryption process isbypassed. If the condition is true, control is transferred tooperational block 2014. If the test result is false, operational block2015 transfers the decryption key information from the scratchpad memory1405 into the DES module 1407.

Operational block 2011 sets the MEMADDR register to the value of CPIO toindicate which location in memory 1405 should be read.

Operational block 2016 reads the contents of memory 1405 as described bythe address located in register MEMADDR and transfers the contents toregister CPIO in control processor bus interface 1409.

Operational block 2012 writes the location of memory 1405 at the addressdesignated by MEMADDR with the contents described by register CPIO.

Operational block 2017 transfers the contents of register MEMADDR toregister CPIO to indicate to the control processor 1006 which locationof memory 1405 was written.

Operational block 2014 acknowledges command completion to controlprocessor 1006. Control is returned to the decryptor program routine viaoval 2007.

Returning to conditional branch point 2003, which tests whether datalink J contains an encryption request according to Table 4 ofoperational block 1608 of the PADEC subroutine shown in FIG. 16. If thetest result in step 2003 is true, operational block 2018 clears theencryption request and proceeds through conditional branch point 2019.

Conditional branch point 2019 tests whether the encryption mode is tobypass encryption. If the test result in step 2019 is true, conditionalbranch point 2020 tests which cipher text block located in scratchpadmemory 1405 (FIG. 14) is to be cleared to 0. If the test result in step2020 is false, operational block 2021 clears the cipher text block 1 inscratchpad memory 1405. If the test result in step 2020 is true,operational block 2022 is entered which clears the cipher text block 0in scratchpad memory 1405.

Returning to conditional branch point 2019, which tests whether theencryption mode is to bypass encryption. If the test result is false,operational block 2023 is entered, which transfers the plain textcounter contents located in memory 1405 into the DES module 1407 (FIG.14).

Operational block 2024 instructs the DES module 1407 to perform thecipher rounds, i.e., the encryption of the plain text.

Conditional branch point 2025 tests which cipher text block located inscratchpad memory 1405 (FIG. 14) is to be loaded with the DES modulecipher text. If the condition is true, operational block 2026 is enteredwhich transfers the cipher text from the DES module into the cipher textblock 0. If the test result in step 2025 is false, operational block2027 is entered which transfers the cipher text from the DES module intothe cipher text (CT) block 1.

Then, control is returned to the decryptor program routine shown in FIG.15 via oval 2028.

Thus, the CENCHR routine controls the operation of DEC 1002 to effectcommon time shared operation of DES module 1407 to generate in responseto encryption requests from PADEC (Table 4) and utilizing the suppliedplain text data from PADEC the corresponding cipher text data that isused to decrypt the incoming audio samples.

Audio Bridge

Details of audio bridge 1003 of FIG. 10 are shown in simplified blockdiagram form in FIG. 22. Audio bridge 1003 includes digital signalprocessor (DSP) 2201, digital signal processor (DSP) 2202, hostprocessor 2203, program memory 2204, output register 2205, programmemory 2206, output register 2207, interrupt controller 2208, timermodule 2209, host processor memory 2210 and control processor interface2211. Units 2204 through 2211 are interconnected to host processor 2203via bus 2212. Program memory 2204 and output register 2205 areassociated with DSP 2201 while program memory 2206 and output register2207 are associated with DSP 2202. Program memories 2204 and 2206 appearas dual port storage systems. DSPs 2201 and 2202 have read-only accessto memories 2204 and 2206, respectively, for fetching instructions andprogram constants. Host processor 2203 has read-write access to memories2204 and 2206 in order to down-line load the DSP programs and to modifycertain of the memory locations that appear as constants. DSPs 2201 and2202 communicate via output registers 2205 and 2207, respectively, tohost processor 2203. DSP 2201 under program control can direct itsoutput to output register 2205 or to DSP 2202. Similarly, DSP 2202 candirect its output to output register 2207 or via 1015 to audio encryptor1004. Each of DSPs 2201 and 2202 includes RAM memory which as describedbelow stores audio input samples AU and also processed audio outputsamples PAU. It is noted that two DSPs, i.e., DSP 2201 and DSP 2202, areemployed in this embodiment to enhance the audio bridge throughput,i.e., speed up the audio processing. To this end, DSP 2201 processessamples represented by bits A9 through A16 while DSP 2202 processessamples represented by bits A1 through A8. Anyone of a number of digitalsignal processing (DSP) units known in the art may be employed in audiobridge 1003. One such DSP unit is described in several articles in TheBell System Technical Journal, Vol. 60, No. 7, Part 2, dated September1981 and manufactured by Western Electric Company. Similarly, hostprocessor 2203 may be any of the known microprocessors. In thisembodiment a Motorola MC 6809 microprocessor unit is employed.

It is important to note that there is no fixed assignment of the VTSlinks to conference rooms and/or other MLCTs. Accordingly, audio bridge1003 under program control effects, in accordance with an aspect of theinvention, programmable audio mixing of the incoming audio signals fromthe VTS links. Additionally, it is possible to controllably reconfigurean on going conference without disrupting it. To this end, audio bridge1003 processes the incoming audio from the rooms in the conference, inaccordance with an aspect of the invention, by employing a partial sumalgorithm which employs matrix multiplication to insure the appropriateaudio mix is sent to the rooms in the conference. Use of the partial sumarrangement is important to obtain the desired audio mixes when morethan one MLCT is used in a conference.

Audio bridge 1003 of FIG. 22 must controllably process the audioinformation obtained from rooms serviced by the MLCT and also the audioinformation from rooms serviced by one or more remote MLCTs. Since morethan one MLCT may be in a conference and since MLCT's communicate to oneanother over one out-going data link a straight summation of the audioinformation cannot readily be realized. This problem of audio mixing isovercome, in accordance with an aspect of the invention, byadvantageously using a partial summing process in each MLCT. To thisend, the audio information from rooms serviced by a particular MLCT aresummed locally and transmitted as a partial sum to the other MLCTs inthe conference. The partial sums from the other MLCTs are controllablyselectively combined with the locally generated partial sum to generatethe appropriate audio mixes for the rooms serviced by the MLCT.

This summing is realized, in accordance with an aspect of the invention,by control processor 1006 (FIG. 39) under program control a programmablematrix operation which in this example employs two NXN matricesdetermined according to the MLCT in-links assignments and the desiredaudio mixes, namely, ##EQU1## Where the processed audio version PAU1 forout-link J is generated via matrices 1 from the in-link audio (AU) fromthe appropriate in-links. The process performed by DSP 2201 and DSP 2202is ##EQU2## where PAU represents the processed audio information, J isthe out-link, I is the in-link and AU is the in-link audio information.Constants K, I, and J are determined by control processor 1006 from thelink assignments provided by processor 1006 and a noise guard algorithmprovided from audio bridge 1003 host processor 2210 (FIG. 22).

The first matrix provided from (1) is called a NO-LOSS matrix and thesecond is called a LOSS matrix. The NO-LOSS matrix contains either 0.0or 1.0 as elements K depending on the conference link assignments, i.e.,which rooms are active, namely,

    KNOLOSS[I, J]=KASSIGN[I, J],                               (3)

and

    KLOSS[I, J]=KNOLOSS[I, J]·L[I]                    (4)

where L[I]=-12db except when the particular link is a MLCT in-link.Then, L [I]=1.0 or 0db.

The noise guard algorithm described below chooses appropriate columns Kfrom the NOLOSS matrix or the LOSS matrix depending on whether there isaudio or no audio on a given in-link I. The constants K from the columnchosen by control processor 1006 are provided to DSP 2201 and 2202 formultiplication with the incoming audio information AU and, therefore,generating the audio mixes as described below. This allows for thecontrollable insertion of loss in the audio information in those linksin which no speech is present to attenuate noise.

Thus, from matrix (3) it is seen that by appropriate selection ofconstant K any weighted sum of the input audio samples AU can begenerated for each of the processed audio output samples PAU.Consequently, the partial audio sums to be transmitted to other MLCT andalso the audio mixes for each conference location are readily obtainableunder program control. Additionally, from gain matrix (4) it is seenthat any predetermined loss value, can be readily inserted in any of theaudio input signals AU. Moreover, if desired any predetermined gainvalue is also readily inserted in any of the audio input signals AU. Inthis example, not to be construed as limiting the scope of the inventiona loss of -12db is controllably inserted as desired.

As shown in the flow charts and described below audio bridge 1003operates under program control to generate the desired processed audiosamples. To this end, the incoming audio samples are converted frommu-law PCM into linear form. The linear samples from a currentlyreceived frame are stored in a first buffer memory, e.g., ABUFA[I],while the linear samples from the last previous frame are stored in asecond buffer memory, e.g., ABUFB[I]. While inputting audio samples intoABUFA[I], the audio samples in ABUFB[I] are processed via a matrixmultiplication, in accordance with an aspect of the invention, togenerate desired partial sum processed audio samples for each locationin the conference, as well as, the appropriate partial summation oflocations served by the MLCT for transmission to other MLCTs in theconference, if any. When inputting samples into ABUFB[I], the samplesstored in ABUFA[I] are being processed. The processed samples areconverted from linear to mu-law PCM and supplied to encryptor 1004 forfurther processing.

FIGS. 23, 24, 25, and 26 connected as shown form a flowchart of aprogram stored in program memory 2204 for controlling operation ofdigital signal processor 2201 (FIG. 22). It is noted that the referencesto registers in the following flow charts are registers in the RAMmemory of either DSP 2201 or DSP 2202.

Accordingly, the program routine is entered via oval 2301. Thereafter,operational block 2302 initializes the digital signal processorinterface (not shown in FIG. 22).

Operational block 2303 sets counter J=1.

Operational block 2304 initializes the register ABUFA containing audioinformation for port J to 0.

Operational block 2305 initializes the audio energy accumulator MGRDassociated with port J to a constant value of -NTHSLD.

Operational block 2306 increments the counter J, i.e., sets J=J+1.

Conditional branch point 2307 tests whether counter J is greater than N.If the test result is false, control is returned to operational block2304 and steps 2304-2307 are iterated until step 2307 yields a trueresult. Then, operational block 2308 sets counter SMPLCT to the valueNSMPL.

Conditional branch point 2309 tests for the start of a DS-1 frame, i.e.,for the DS-1 frame sync. If the test result is false, step 2309 loopsuntil a true test result is obtained.

Then, operational block 2310 sets counter J=1.

Operational block 2311 sets counter I=1 and the audio accumulation sumregister ASUM=0.

Operational block 2312 sets the audio accumulation sum to its previousvalue plus the audio associated with port I after multiplying by amatrix element K[I,J] located in the data memory associated with thedigital signal processor 2201 of FIG. 22.

Operational block 2313 increments the counter I, i.e., sets I=I+1.

Conditional branch point 2314 tests whether I>N. If the test result isfalse, control is returned to operational block 2312 and steps 2312,2313 and 2314 are repeated until the test result in conditional branchpoint 2314 is true. When the test result in step 2314 is true,operational block 2315 is entered to receive audio informationassociated with port J (i.e., bits A-1 through A-8) from audio decryptor1002 via link 1013 in FIGS. 14 and 22 and stores it in register AEVENassociated with port J.

Operational block 2316 performs a linear to mu transformation on thecontents of the audio accumulation contained in ASUM previously computedin operational block 2312 and stores it in register PAODD.

Operational block 2317 sends audio information, i.e., bits A-1 throughA-8 previously stored in register AEVEN in step 2315 to digital signalprocessor 2202.

Operational block 2318 receives audio information, i.e., bits A-9through A-16 from audio decryptor 1002 via link 1013 in FIG. 14 andperforms a mu to linear transformation and then stores the result inregister ABUFB associated with port J.

Operational block 2319 sends to digital signal processor 2302 thecontents of register PAODD which were computed in step 2316.

Operational block 2320 increments counter J, i.e., sets J=J+1. Thus,steps 2310 through 2320.

Conditional branch point 2321 tests whether J>N. If the test result isfalse, control is returned to operational block 2311 in which case steps2311-2321 are iterated until step 2321 yields a true result. Thenoperational block 2322 sets counter J=1.

Operational block 2323 causes the absolute value of register ABUFAassociated with port J to be added to the absolute value of registerABUFB associated with port J. The result of the addition is multipliedby a predetermined constant AVGWT. The result of the multiplication isadded to the value in register NGRD associated with port J.

Operational block 2324 increments counter J, i.e., sets J=J+1.

Conditional branch point 2325 tests whether J>N. If the test result isfalse, control is returned to operational block 2323 and step 2323-2325are iterated until step 2325 yields a true result.

Steps 2326-2338 are substantially identical to steps 2309-2321 exceptfor several differences, namely, steps 2329 and 2335. In step 2329,ABUFB associated with J is utilized instead of ABUFA and in step, 2335ABUFA associated with port J is utilized instead of ABUFB. Operationalblock 2339 decrements counter SMPLCT.

Conditional branch point 2340 tests if counter SMPLCT is equal to 0. Ifthe test result is false, control is returned to conditional branchpoint 2309. If the test result in step 2340 is true, operational block2341 is entered which sets counter SMPLCT to a predetermined valueNSMPL.

Operational block 2342 sets counter J=1 and sets the active audio energyregister AENGY=0.

Operational block 2343 causes information to be supplied to hostprocessor 2203 indicating which ports have active audio information. Inthe event that a port has audio information, an associated bit inregister AENGY is set to be a logical 1.

Operational block 2344 sets the audio energy accumulator NGRD associatedwith port J to a predetermined value -NTHSLD.

Operational block 2345 increments counter J, i.e., sets J=J+1.

Conditional branch point 2345 tests whether J>N. If the test result isfalse, control is returned to operational block 2343 and steps 2343-2345are repeated until step 2345 yields a true result. Then, operationalblock 2346 sends the active audio register AENGY contents to the hostprocessor 2203 via output register 2205 and link 2212. Thereafter,control is returned to conditional branch point 2309.

FIGS. 27, 28, 29 and 30 connected as shown form a flowchart of a programroutine stored in program memory 2206 for controlling digital signalprocessor 2202. Steps 2701-2746 of the program are substantiallyidentical to steps 2301-2346 of the program routine shown in FIGS. 23-26and described above. The differences are that the program routine shownin FIGS. 23-26 causes digital signal processor 2201 to pass audio bitsA-1 through A-8 to digital signal processor 2202 and causes audio bitsA-9 through A-16 to be mixed according to steps 2315-2320 (FIGS. 23-26).On the other hand, the program routine shown in FIGS. 27-30 passesthrough mixed audio bits A-9 through A-16 from digital signal processor2201 and mixes audio bits A-1 through A-8 as described in steps2715-2720. Accordingly, the remaining steps of the program routine shownin FIGS. 27-30 will not be described in detail again.

FIG. 31 is a flowchart of a DSP control (DSPCTL) program task stored inhost processor memory 2210 for operating host processor 2203 undercontrol of a standard operating system to interact with digital signalprocessors 2201 and 2202.

Accordingly, the DSPCTL task program routine is entered via oval 3101.Thereafter, operational block 3102 sets counter J=1.

Operational block 3103 sets noise guard holdover counter NGHCT equal toa predetermined value HOLDVR associated with port J.

Operational block 3104 increments counter J, i.e., sets J=J+1.

Conditional branch point 3105 tests whether J>N. If the test result isfalse, control is returned to operational block 3103 and, thereafter,steps 3103, 3104 and 3105 are repeated until the test result is true.

Operational block 3106 places both Digital Signal Processor 2201 andDigital Signal Processor 2202 in the run state.

Oval 3107 places a call to the operating system executed by hostprocessor 2203 to suspend the operation of program task DSPCTL untildigital signal processor 2201 has sent output data to output register2205.

Operational block 3108 calls a subroutine NGRD to perform the noiseguard algorithm from audio energy samples associated with audio bits A-9through A-16. A flowchart of the NGRD Subroutine is shown in FIG. 32 andis described below.

Oval 3109 places a call to the operating system executed by hostprocessor 2203 to suspend the operation of the program task DSPCTL untildigital signal processor 2202 has sent data to output register 2207associated with audio energy of the incoming audio information.

Operational block 3110 calls subroutine NGRD to perform the noise guardalgorithm from audio energy samples associated with audio bits A-0through A-8 as described below.

Control is returned to oval 3107 and steps 3107-3710 are repeatedcontinuously in an endless loop.

FIG. 32 shows a flowchart of the noise guard program subroutine (NGRD)utilized in the DSPCTL program task of FIG. 31. Accordingly, the NGRDsubroutine is entered via oval 3201. Thereafter, operational block 3202sets counter I=1.

Conditional branch point 3203 tests to determine whether audio energy ispresent from link I. If the test result is true, operational block 3204sets the noise guard holdover counter associated with port I equal tothe predetermined value HOLDVR. If the test result in step 3203 isfalse, conditional branch point 3205 tests whether the noise guardholdover counter NGHCT associated with port I is equal to 0. If the testresult is false, operational block 3206 will decrement counter NGHCTassociated with port I.

Operational block 3207 sets counter J=1.

Operational block 3208 transfers to digital signal processor programmemory 2204 and program memory 2206 (FIG. 22) column J of matrix KNOLOSSassociated with port I.

Operational block 3209 sets counter J=J+1. Conditional branch point 3210tests whether counter J>N. If the result is false, control is returnedto operational block 3208 and steps 3208-3210 are repeated until thestep 3210 yields a true result and control is transferred to operationalblock 3211.

Returning to conditional branch point 3205 if the test result is true,i.e., counter NGHCT=0 operational block 3213 sets counter J=1 indicatingthat loss is to be inserted.

Operational block 3214 transfers to digital signal processor programmemory 2204 and 2206 (FIG. 22) column J of matrix KLOSS associated withport I.

Operational block 3215 sets counter J=J+1 and conditional branch point3216 tests whether J>N and if the test results are false, control isreturned to operational block 3214 and steps 3214-3416 are repeateduntil step 3216 yields a true test result. Then, control is transferredto operational block 3211.

Operational block 3211 sets counter I=I+1. Conditional branch point 3212tests whether I>N. If the test result is false, control is returned toconditional branch point 3203 and steps 3203-3211 are repeated asdescribed above until step 3212 yields a true result. Thereafter,control is returned to program task DSPCTL of FIG. 31 via oval 3217.

Audio Encryptor (ENC)

Details of audio encryptor (ENC) 1004 of FIG. 10 are shown in simplifiedblock diagram form in FIG. 33. ENC 1004 provides, in accordance with anaspect of the invention, a common control arrangement for encrypting amultiplicity of VTS audio channels. Accordingly, shown aremicrocontroller 3301 and associated program memory 3302, BOC data outputport 3303, BOC data write control port 3304, scratchpad RAM memory 3305,serial input port 3306, DES data encryption standard module 3307,control channel output port 3308 and control processor bus interface3309. Microcontroller 3301 is interconnected with units 3303 through3309 via bus 3310. Additionally, microcontroller 3301 receives a resetsignal from interface 3309 and code instructions from program memory3302 in response to address signals supplied thereto. Program memory3302 is also connected to interface 3309. In this example,microcontroller 3301 is a Signetics 8×305 and module 3307 includes aFairchild 9414-1,-2,-3,-4 DES chip set which implements the NationalBureau of Standards (NBS) Data Encryption Standard (DES).

ENC 1004 receives the conferenced audio from audio bridge 1003 via 1015and 1016 and control channel information, namely, C1 through CN fromcontrol processor 1006 via 1010-1 through 1010-N, respectively, for allN outgoing VTS channels. Microcontroller 3301 under program controlcauses the cipher text and audio decipherment bits (D-bits) for each VTSchannel to be generated as shown in FIG. 12. After an encryptionoperation on all N audio channels, the control channel information, theencrypted audio and the generated audio decipherment bits are combinedinto the time slot groups and supplied via 1017 and 1018 to BOC 1005 fortransmission to cross connect 102. Communication to BOC 1005 is achievedthrough BOC data output port 3303 and BOC data write output port 3304.BOC data write output port 3304 contains the time slot number on whichthe respective data present in BOC data output port 3303 should beinserted at the output data links of BOC 1005.

Control processor 1006 accesses ENC 1004 via control processor interface3309. Interface 3309 provides read-write access to microcontrollerprogram memory 3302 for loading the encryptor program, for verifyingthat the program is stored properly, and for halting and restartingmicrocontroller 3301 as appropriate. Once microcontroller 3301 is in therun mode, control processor 1006 (FIG. 39) communicates withmicrocontroller 1401 via bus 3310. Additionally, control processor 1006(FIG. 39) supplies the DES key and the encryption modes, namely,encrypted/clear via interface 3309 to ENC 1004.

Moreover, ENC 1004 realizes common control encryption of the mixed audiosignals under program control.

Operation of audio encryptor (ENC) 1004 (FIG. 33) is controlled bymicrocontroller 3301 under control of programs stored in program memory3302.

FIGS. 34, 35 and 36 connected as shown form a flowchart of a programroutine stored in program memory 3302 to effect the common controlencryption of the mixed audio information from audio bridge 1003 and toinsert control channel information from control processor 1006 (FIG.39).

Accordingly, the program is entered via oval 3401. Thereafter,operational block 3402 initializes system variables to known states, forexample, ENCMODE is set to bypass encryption if transmission is in theclear, the counter DBCNTR is set equal to 0, and the Hamming code tableis initialized, among others.

Conditional branch point 3403 tests whether encryption should beperformed. If the test result is false, control is transferred tooperational block 3408 where cipher text CPHRTXT in scratchpad memory3305 is set to 0. If the test result in step 3403 is true, conditionalbranch point 3404 tests whether the least two significant bits ofcounter DPCNTR are both equal to 0. If the result is true, operationalblock 3405 transfers the present state of the plain text countercontents PRPTXT located in memory 3305 to the DES module 3307.

Operational block 3406 instructs the DES module 3307 to perform thecipher rounds, i.e., the encryption of the plain text entered in step3405.

Operational block 3407 extracts from DES module 1407 two 8-bit words outof 8 cipher words generated by the DES module 3307 and stores them inregister CPHRTXT located in memory 3305.

Returning to step 3404, if the test result is false, i.e., either of thetwo least significant bits counter DBCNTR being equal to 1, control istransferred to operational block 3407 as described above.

Conditional branch point 3409 tests whether the four least significantbits of counter DBCNTR are equal to 0. If the test result is true, theD-bit counter represents the index n as shown in VTS Data Format: AudioDecipher Group of Table 3 described above multiplied by 16.

Operational block 3410 extracts the four-bit block associated with indexn from register PRPTXT in memory 3305, computes the Hamming code checkbits and stores the resulting 8-bit code word in register LRTNP inmemory 3305. If the test result in step 3409 is false, control istransferred directly to conditional branch point 3411.

Conditional branch point 3411 tests for the occurrence of the DS-1 framesync. If the test result is false, conditional branch point 3412 testswhether control processor 1006 has requested execution of a command. Ifthe test result of step 3412 is false, control is transferred toconditional branch point 3413 which tests for the occurrence of the DS-1frame sync, i.e., the beginning of the DS-1 frame. If the test result instep 3413 is false, the step is reexecuted until the test yields a trueresult. Thereafter, control is transferred to operational block 3425.

Returning to conditional branch point 3411, if the test result, i.e.,check for DS-1 frame sync, yields a true result control is transferreddirectly to operational block 3425.

Operational block 3425 causes the mixed audio information associatedwith each VTS link from audio bridge 1003 to be read via serial inputport 3306 (FIG. 33) and transmitted to audio encryptor 1004 via links1015 and 1016 (FIG. 33). The audio information so read is stored inappropriate locations in memory 3305.

Operational block 3426 sets counter J=1. Operational block 3427 readsthe control channel information, i.e., bit CJ supplied to audioencryptor 1004 from control processor 1006 via one of the N controlchannel links, namely, links 1010-1 through 1010-N. This read operationis performed on control channel input port 3308 (FIG. 33).

Operational block 3428 writes into register BOTSAR of BOC data writecontrol port 3304 (FIG. 33) time slot address information for associatedport J.

Operational block 3429 sets register ABUFADR with the address of amemory location in memory 3305 associated with a location containing theaudio information associated with outgoing link J.

Operational block 3430 calls the PAENC subroutine to perform audioencryption and to insert the control channel information into theassociated port J time slot group in BOC 1005. A flowchart of the PAENCsubroutine is shown in FIG. 37 and described below.

Operational block 3431 increments counter J, i.e., sets J=J+1.

Conditional branch point 3432 tests whether J>N. If the test result istrue, control is returned to operational block 3427 and steps 3427-3432are repeated until the test in step 2232 yields a true result. Thiseffects, in accordance with an aspect of the invention, the commoncontrol encryption of the audio information associated with all of theoutgoing links.

Operational block 3433 increments D-bit counter DBCNTR.

Conditional branch point 3434 tests whether the D-bit counter is equalto 128, i.e., whether it is at the end of the D-bit superframe. If thetest result is true, operational block 3435 transfers the contents ofthe next state plain text counter NXTPTXT to register PRTTXT in memory3305 (FIG. 33).

Operational block 3436 increments the counter NXTPTXT and, thereafter,control is returned to conditional branch point 3403. If the test resultin step 3434 is false, control is transferred directly to conditionalbranch point 3403.

Returning to conditional branch point 3412, if the test result thereinis true, i.e., if a command execution request is present from controlprocessor 1006, control is transferred to operational block 3414.

Operational block 3414 reads the command from control processor businterface 3309 (FIG. 33) and, then, oval 3415 causes one of severaloperations to be effected depending on the type of command which wasread in step 3414. The commands are, for example, read encryption mode,set encryption mode, read memory 3305 and write memory 3305. Theoperations effecting these commands are realized by operational blocks3416, 3417, 3418 and 3419, respectively.

Operational block 3416 transfers the encryption mode located in theENCMODE register in memory 3305 to register CPIO located in controlprocessor interface 3309 (FIG. 33).

Operational block 3417 sets register ENCMODE to the value in registerCPIO.

Conditional branch point 3420 tests whether the encryption process is tobe bypassed. If the test result is true, control is transferred tooperational block 3424. If the test result is false, operational block3421 transfers the encryption key information from memory 3305 into theDES module 3307.

Operational block 3418 sets register MENADDR to the value in CPIO toindicate which location in memory 3305 should be read.

Operational block 3422 reads the contents of memory 3305 identified bythe address in register NEMADDR and transfers the read contents toregister CPIO in control processor bus interface 3309.

Operational block 3419 writes memory 3305 at the location identified bythe address designated by register MENADDR with the contents in registerCPIO.

Operational block 3423 transfers the contents of register MENADDR toregister CPIO to indicate to control processor 1006 which location ofmemory 3305 was written.

Operational block 3424 acknowledges completion of the requested commandto control processor 1006. Thereafter, control is returned toconditional branch point 3413.

FIG. 37 is a flowchart of the PAENC subroutine utilized in step 3430 ofroutine ESCVC described above in relationship to FIG. 34. Accordingly,the PAENC subroutine is entered via oval 3701.

Operational block 3702 encrypts audio bits A-1 through A-16 addressed bythe contents of register ABUFADR by performing an exclusive-OR operationwith the contents of register CPHRTXT in memory 3305 (FIG. 33).

Operational block 3704 increments the time slot register BOTSAR locatedin BOC data write control port 3304 (FIG. 33).

Operational block 3705 writes encrypted audio bits A-4 through A-10 intoBOC 1005.

Operational block 3706 increments the time slot register BOTSAR locatedin the BOC data write control port 3304.

Operational block 3707 writes encrypted audio bits A-11 through A-16into BOC 1005.

Operational block 3708 implements a 128 way branch based on the state ofthe least 7 significant bits of D-bit counter DBCNTR and operationalblock 3709 implements a set of operations based on each of the 128branch points to choose a proper state of the D-bit. By way of example,if D-bit counter DBCNTR=1, bit L-1 from register LRTNP is chosen. IfD-bit counter DBCNTR=2, D=S1=1. See FIG. 12 and Table 3 for therelationship of the remaining D-bits to the L, S and P bits.

Operational block 3710 writes the D-bit information into BOC 1005 andthen control is returned to the encryptor program routine (FIG. 34) viaoval 3711.

Bridge Output Circuit (BOC)

Details of bridge output circuit (BOC) 1005 are shown in simplifiedblock diagram form in FIG. 38. The function of BOC 1005 is to combinethe subset of the video control bits, namely, bits FD, X, X stored invideo latch 1103 within the present 125 u sec frame in bridge inputcircuit (BIC) 1001 (FIG. 11) with the audio information provided byencryptor 1004 (FIG. 33), and to provide the combined information atpreselected fixed time slots to appropriate ones of time multiplexedswitches 601-1 through 601-N in cross connect 102 (FIG. 6) in accordancewith the format set out in Table 3 above.

Accordingly, shown in FIG. 38 are data selectors 3801-1 through 3801-N,which form cross connect signal formatter 3810 dual port control memory3802, dual port data memory 3803, idle time slot code generator 3804,time slot counter 3805 and control processor interface 1606. Videosignaling bits FD, X, X are supplied from BIC 1001 via 1008-1 through1008-N to data selectors 3801-1 through 3801-N, respectively. Datamemory 3803 is accessed by encryptor 1004 via 1017 and 1018. Memory 3803includes a RAM having a duplicate sets of memory locations. A first setof the memory locations is written with the processed audio and controlchannel information from encryptor 1004 during a current frame while asecond set of memory locations is being read to supply the informationwritten during a previous frame to appropriate ones of data selectors3801-1 through 3801-N. During a subsequent frame, the first set ofmemory locations is read while the second set is written with theinformation from encryptor 1004. Data selector 3801-1 through 3801-Ncombine on a per-time-slot and per-bit basis the video signaling bitssupplied from BIC 1001 and the audio and control channel informationsupplied from data memory 3803. The combination is in accordance withcontrol codes stored in control memory 3802 which are supplied viainterface 3806 from control processor 1006 (FIG. 39). An idle time slotcode generated by idle time slot generator 3804 is inserted in unusedtime slots. Both data memory 3803 and control memory 3802 aresequentially addressed by time slot counter 3805 which includes amodulo-256 binary counter synchronized to the internal frame time-slotboundaries. Control processor 1006 with read-write access to anylocation in control memory 3802 for setting up the desired informationselection pattern. Once a selection pattern is written in control memory3802, no other interaction is required unless a VTS port and, hence, aconference room or another MLCT is added to or deleted from theconference. Accordingly, the combined processed information isselectively supplied to cross connect 102 via 109-1 through 109-N.

Each code word stored in control memory 3802 includes 8 bits and isassociated with a specific time slot in the cross connect frame formatshown in FIG. 3. Four of the bits are not used and two bits are used forparity. The remaining two bits are operational control bits and are usedto indicate to the switching network which of the 256 time slots of theN VTS channels are used to send processed audio and control channelinformation and, further, to indicate which time slots should be filledwith the video signaling bits (FD,X,X). The two operational control bitsare defined as follows:

b4: TDSEN. Transmit Data Slot Enable. A logical 1 indicates that theassociated time slot is MAPPED and the data contained in the respectivedata RAM address in data memory 3803 is formatted with odd parity issent to the switch network. A logical 0 indicates that the time slot isidle and an IDLE code of all logical 1's is supplied from idle codegenerator 3804 and even parity is sent.

b5: TVCSEN. Transmit Video Control Slot Enable. A logical 1 indicatesthat the video signaling information (FD,X,X) from BIC 1001 should beinseted in the respective time slot if the time slot is MAPPED.

In this example, data selectors 3801-1 through 3801-N are controlled viacontrol memory 3802 to insert the video control signals from bridgeinput circuit 1001 into the appropriate bit positions of a time slot.Similarly, data selectors 38011-1 through 3801-N are also controlled toinsert the processed audio information from audio encryptor 1004 intothe fixed time slots of the composite signals as shown in FIG. 9.Accordingly, the video control, c-bit and processed audio are insertedfor VTS port 500-1 into time slots 32, 33 and 34, for VTS port 500-2into time slots 36, 37 and 38, for VTS port 500-3 into time slots 40, 41and 42, for VTS port 500-4 into time into time slots 44, 45 and 46, forVTS port 500-5 into time slots 48, 49 and 50 and for VTS port 500-6 intotime slots 52, 53 and 54.

Bridge Control Processor (CP)

Details of control processor 1006 of FIG. 10 are shown in simplifiedblock diagram form in FIG. 39. Accordingly, shown are central processingunit (CPU) 3901, interrupt controller 3902, timer module 3903, memory3904, microprocessor system interface 3905, control processor businterface 3906 and control channel interface 3907, all of which areinterconnected via bus 3908. CPU 3901 is a microprocessor unit. In thisembodiment a Motorola MC6809 unit is used. Memory 3904 includes bothread- only memory (ROM) and read-write memory (RAM). Control processorbus inter- face 3906 operates as a buffer and provides all interruptcontrol from bus 1007 to interrupt controller 3902. Interrupt controller3902 resolves priority of the various interrupts and, then, interruptsCPU 3901. CPU 3901 in response to the interrupt executes the requiredtask to service the interrupt. Timer module 3903 is a programmable timerwhich generates a "realtime" clock interrupt for CPU 3901 to coordinateall time-referenced tasks. For example, the implementing of timingparameters in the control of conference video selection dynamics.Microprocessor system 101 interface 3905 provides a standardcommunication interface with microprocessor system 101.

Control channel interface 3907 includes a set of N high-level data linkscontrollers (HDLCs) (not shown) each associated with one of controlchannels C, through CN to provide the basic frame structure of the BX.25 level 2 auxiliary control channel. The MC6854 which is commerciallyavailable from is used for this purpose. Interface 3907 also includes aset of N direct memory access (DMA) controllers (not shown) associatedon a one-to-one basis with control channels C1 through CN which areemployed to unburden CPU 3901 of such tasks as transferral of theauxiliary channel messages from and to memory 3904 to the HDLCs incontrol channel interface 3907. Tasks left to CPU 3901 are "high-level"decision tasks and the generation of messages of actual messages to betransmitted via auxiliary channels C, through CN. The HDLCs handle suchtasks as data transferral to memory 3904 through direct memory access(DMA) operations. Upon receipt of a complete HDLC frame on any ofauxiliary data channels C1 through CN control channel interface 3907interrupts CPU 3901 and an appropriate task is rescheduled to handle thereceived message. In transmitting auxiliary channel messages overauxiliary data channels C1 through CN, control channel interface 3907under control of CPU 3901 retrieves a prepared message from memory 3904and employs DMA operations to transmit the message via an appropriateone of auxiliary channels C1 through CN to a conference room or toanother MLCT.

Control processor 1006 under program control effects operation of theMLCT to realize the desired video switching, audio mixing, conferenceset up and conference reconfiguration as desired and describedhereinafter. Accordingly, control processor 1006 is employed underprogram control to effect all set up operations within bridge 103 and toeffect all communications to rooms and other MLCTs in the conference inorder to realize all communication and video control operations in theMLCT. To this end, control processor 1006 operations under an operatingsystem of a type well known in the art to realize the desired operationsunder control of specific program routines to be described hereinafter.It is noted, that control processor 1006 does not directly make thevideo switches. However, control processor 1006 makes all the decisionsrelating to the video information to be viewed by each room in theconference. Once the selection decisions have been made, they arecommunicated to microprocessor system 101 which, in turn, controlscross-connect 102 to make the switches as described above.

Communication to audio decryptor (DEC) 1002, audio bridge 1003 and audioencryptor (ENC) 1004 is performed by control processor 1006 task XPCTL.The XPCTL task is responsible to report audio muting informationgenerated by the audio decryptor microcontroller 1401 (FIG. 14) to theinput link status array located in the control processor memory 3904(FIG. 39) for use by the conferencing tasks described below.

Other functions supported by the XPCTL task include the generation ofboth the KLOSS[I,J] and the KNOLOSS[I,J] matrices for use by audiobridge 1004 host processor 2203 (FIG. 22) to indicate how the partialaudio summations should be performed in accordance with the respectiveinput VTS link assignments in audio bridge 1003. Furthermore, the XPCTLenters the DES encryption and decryption key into both audio decryptormemory 1405 (FIG. 14) and audio encryptor memory 3305 (FIG. 33) and tocommunicate the encryption mode to them, i.e. bypassencryption/decryption or activate encryption/decryption.

FIG. 40 is a flowchart of a program routine stored in memory 3904 of thecontrol processor 1006 to effect the communication and control functionsdescribed above via the control processor bus interface 3906 and bus3903 of FIG. 39.

Accordingly, the program is entered via oval 4001. Thereafter,operational block 4002 sets event flags ENCFLG=1, XPLAFG=1, andLDECEFG=1. Event flag ENCFLG indicates the occurrence of a newencryption state change event, XPLAFG indicates a new link assignmentevent, and LDECEFG indicates that a input link mute state change eventoccurred in the audio decryptor 1002.

Operational block 4003 calls subroutine PREST which checks whether a newencryption state change event occurred and performs the required stepsto change the encryption key and encryption mode in both the audiodecryptor 1002 and the audio encryptor 1004. The PREST routine is shownin FIG. 41 and described below.

Operational block 4004 calls subroutine PRNLA which checks whether a newlink assignment has occurred and, accordingly, performs the requiredsteps to format the audio mixing matrices. The check result is relayedto the audio bridge 1003. It also relays the new link assignment to thedecryptor 1002 for enabling mute information associated with the newassigned ports to cause a program flow event (interrupt) in the controlprocessor operating system. Conversely, if a link is removed from theconference, the decryptor 1002 will disable the respective link'sability to cause an interrupt on the mute state change in the controlprocessor operating system. Thereby, adding or deleting a room to orfrom, respectively, the conference without disrupting it, in accordancewith an aspect of the invention. The PRNLA routine is shown in FIGS. 42,43, 44 and 45 and described below.

Operational block 4005 calls subroutine PRDEVT which checks whether anew mute state change event occurred at the audio decryptor 1002 andperforms the required steps to relay the new audio muting informationgenerated by the audio decryptor microcontroller 1401 (FIG. 14) to theinput link status array located in the control processor memory 3904 foruse by the conferencing tasks described below. The PRDEVT routine isshown in FIG. 46 and described below.

Oval 4006 issues a request to the control processor 1006 operatingsystem to be put in a WAIT state until a POST event occurs in which casecontrol is given to operational block 4003 and steps 4003 through 4006are executed in an endless loop.

FIG. 41 is a flowchart of the PREST subroutine employed by operationalblock 4003 of the XPCTL program task of FIG. 40.

Accordingly, the PREST subroutine is entered by oval 4101. Conditionalbranch point 4102 tests whether event flag ENCFLG is set to 1. If thetest result is false, control is returned to the calling program routineXPCTL via oval 4112. If the test result is true, operational block 4103sets event flag ENCFLG=0.

Conditional branch point 4104 tests whether the new encryption state isto bypass encryption. If the test is true, operational block 4105 placesthe decryptor in the decryption bypass mode by issuing an appropriatecommand to the decryptor microcontroller 1401 (FIG. 14) as described inthe decryptor program shown in FIG. 15.

Operational block 4106 places the encryptor 1004 in the decryptionbypass mode by issuing an appropriate command to the encryptormicrocontroller 3301 (FIG. 33) as described in the encryptor programshown in FIG. 34 and, then, control is given to the calling programroutine XPCTL via oval 4112.

Returning to step 4104, if the test result is false, Operational block4107 transfers the decryption DES key into the audio decryptorscratchpad memory 1405.

Operational Block 4109 places the DEC 1002 in decryption mode.

Operational Block 4110 transfers the decryption DES key into the audioencryptor scratchpad memory 3305.

Operational Block 4111 places the ENC 1004 in the encryption mode.Control is returned to the calling program routine XPCTL via oval 4112.

FIGS. 42, 43, 44 and 45 connected as shown form a flowchart of the PRNLAsubroutine employed by operational block 4004 of the XPCTL program taskof FIG. 40.

Accordingly, the PRNLA subroutine is entered by oval 4201. Conditionalbranch point 4202 tests whether event flag XPLAFG is set to 1. If thetest result is false, control is returned to the calling program routineXPCTL via oval 4203. If the test is true, operational block 4204 setsevent flag XPLAFG=0.

Operational block 4205 sets counter J=1.

Conditional branch point 4206 tests whether input link J is assigned. Ifthe test result is false, operational block 4207 sets the respectivemask bit associated with port J in the decryptor scratchpad memory 1405register MASK equal to 0 indicating "Mute status change masked". If thetest result is true, operational block 4208 sets the respective mask bitassociated with port J in the decryptor scratchpad memory 1405 registerMASK equal to 1 indicating "Mute status change unmasked".

Operational block 4209 increments counter J, i.e. sets J=J+1.

Conditional branch point 4210 tests whether J>N. If the test result isfalse, control is returned to Conditional branch point 4206 and steps4206 through 4210 are repeated until step 4210 yields a true result.

Steps 4211 through 4250 perform the generation of the audio mixingmatrices to be sent to the audio bridge 1003. Accordingly, operationalblock 4211 initializes several arrays of boolean flags as shown, i.e.sets the N-dimensional array LOCAUD[N]=0, the N-dimensional arrayTOTAUD[N]=0, The N-dimensional array MLCTLK[N]=0, and the arrayMAUDCF[N]=0. The Jth element of the LOCAUD array indicates that link Jis assigned to a Room and its audio should contribute to the partialsummation of audio to be sent to other MLCT(s) in the conference. TheJth element of the TOTAUD array indicates that input link J contributesto the total audio sum of all the conference rooms in the conference.The Jth element of the MLCTLK array indicates whether the link isoriginating at another MLCT (if set to 1) or a room (if set to 0). TheKth element in array MAUDCF indicates whether the Kth MLCT in theconference has contributed audio at least once in the case when multiplebroadcast links are assigned to a particular MLCT in the conference.

Operational block 4212 sets counter J=1.

Conditional branch point 4213 tests whether input link J is assigned. Ifthe test result is false, control is given to step 4221. If the testresult is true, conditional branch point 4214 tests whether the assignedinput link is assigned to a room. If the test result in step 4214 istrue, operational block 4215 sets the Jth element of array LOCAUD and ofarray TOTAUD to 1. If the test result is false, i.e. link J is assignedto an MLCT, operational block 4216 sets MLCTLK[J]=1.

Operational block 4217 sets register K with the numerical value of theMLCT number that the Jth link is assigned to.

Conditional branch point 4218 tests whether MLCT K has alreadycontributed to the total audio summation. If the result in step 4218 isfalse, operational block 4219 sets MAUDCF[K]=1 and operational block4220 sets the Jth element of array TOTAUD[J]=1.

Returning to step 4218, if the test result of conditional branch point4218 is true, steps 4219 and 4220 are skipped and control is given tooperational block 4221.

Operational block 4221 increments counter J, i.e. J=J+1.

Conditional branch point 4222 tests whether counter J is greater than N.If the test result is false, control is given to conditional branchpoint 4213 and steps 4213 through 4222 are repeated until step 4222yields a true result.

Operational block 4223 sets counter J=1.

Conditional branch point 4224 tests whether output link J is assigned.If the test result is false, operational block 4236 sets counter I=1.

Operational block 4237 sets the matrix element KNOLOSS[I,J] and thematrix element KLOSS[I,J] in the audio bridge host memory 2210 to 0.0,i.e. -infinite db.

Operational block 4237 increments counter I, i.e. sets counter I=I+1.

Conditional branch point 4239 tests whether I>N. If the test result isfalse, control is returned to operational block 4237 and steps 4237through step 4139 are repeated until conditional branch point 4239yields a true result.

Returning to conditional branch point 4224, If the test result is true,conditional branch point 4225 tests whether output link is assigned to aroom. If the test result is true, operational block 4226 sets counterI=1.

Conditional branch point 4227 tests whether I=J. If the test result istrue, operational block 4233 sets the matrix element KNOLOSS[I,J] andthe matrix element KLOSS[I,J] in the audio bridge host memory 2210 to0.0, i.e. -infinite db. If the test result is false, conditional branchpoint 4228 tests whether the Jth element in array TOTAUD is set to 1. Ifthe result is false, control is given to operational block 4233. If theresult is true, conditional branch point 4230 tests whether the Ith linkis an MLCT link. If the test result in step 4230 is true, operationalblock 4231 sets the matrix element KNOLOSS[I,J] and the matrix elementKLOSS[I,J] in the audio bridge host memory 2210 to 0 db. If the testresult in step 4230 is false, operational block 4232 sets the matrixelement KNOLOSS[I,J] and the matrix element KLOSS[I,J] in the audiobridge host memory 2210 to 0 db and to -12db respectively.

Operational block 4234 sets counter I=I+1.

Conditional branch point 4235 tests whether I>N. If the result is false,control is given to conditional branch point 4227 and steps 4227 through4235 are executed until conditional branch point 4235 yields a trueresult.

Returning to conditional branch point 4225, if the test result is false,operational block 4240 sets counter I=1.

Conditional branch point 4241 tests whether the Ith element in arrayLOCAUD is set to 1. If the test result is false, Operational block 4245sets the matrix element KNOLOSS[I,J] and the matrix element KLOSS[I,J]in the audio bridge host memory 2210 to 0.0, i.e. -infinite db. If theresult is true, conditional branch point 4242 tests whether the Ith linkis an MLCT link. If the test result in step 4242 is true, operationalblock 4243 sets the matrix element KNOLOSS[I,J] and the matrix elementKLOSS[I,J] in the audio bridge host memory 2210 to 0 db. If the testresult in step 4242 is false, operational block 4232 sets the matrixelement KNOLOSS[I,J] and the matrix element KLOSS[I,J] in the audiobridge host memory 2210 to 0 db and to -12db respectively.

Operational block 4246 sets counter I=I+1.

Conditional branch point 4247 tests whether I>N. If the result is false,control is given to conditional branch point 4241 and steps 4241 through4247 are executed until conditional branch point 4247 yields a trueresult.

Operational block 4248 sets counter J=J+1.

Conditional branch point 4249 tests whether J>N. If the result is false,control is given to conditional branch point 4224 and steps 4224 through4249 are repeated until conditional branch point 4249 yields a trueresult in which case control is given to the calling program routineXPCTL via oval 4250.

FIG. 46 is a flowchart of the PRDEVT subroutine employed by operationalblock 4005 of the XPCTL program task of FIG. 40.

Accordingly, the PRDEVT subroutine is entered by oval 4601.

Conditional branch point 4602 tests whether event flag LDECEFG=1. If thetest result is false, control is given to the calling program routineXPCTL via oval 4615. If the test result is true, operational block 4603sets event flag LDECEFG=0.

Operational block 4604 sets a register in control processor memory 3904,namely, NWMUTST=AMUTST by performing an appropriate read from scratchpadmemory 1405 in the decryptor 1002.

Operational block 4605 sets register STSREG in the decryptor scratchpadmemory 1405 with the contents of AMUTST by sending appropriate writecommand to the decryptor microcontroller 1401.

Operational block 4606 sets flag MUTCHGFG=0. MUTCHGFG indicates whetheran audio mute state change occurred.

Conditional branch point 4608 tests whether the audio mute staterecorded in the control processor memory 3904 associated with link J,i.e., MUTST[J], equals the new audio mute state in register NWMUTSTacquired in step 4604. If the test result is false, operational block4609 sets flag MUTCHGFG=1.

Operational block 4610 stores the new audio mute status associated withlink J in an appropriate location control processor memory 3904.

Returning to step 4608, if the test result is true, steps 4609 and 4610are skipped and control is given to operational block 4611 which setscounter J=J+1.

Conditional branch point 4612 tests whether counter J>N. If the testresult is false, control is returned to conditional branch point 4608and steps 4608 through 4612 are repeated until the result in conditionalbranch point 4612 yields a true result.

Conditional branch point 4613 tests whether flag MUTCHGFG was set to 1in the above process. If the test result is true, operational block 4614informs the Facility Administration Module of such event and programcontrol is given to the calling program routine XPCTL via oval 4615. Ifthe test result in conditional branch point 4613 is false, step 4614 isskipped and program control is given to the calling program routineXPCTL via oval 4615.

System Operation

Control Messages

Messages exchanged between the rooms and MLCTs and between MLCTs, i.e.,the VTS message definitions are described below. They are useful inunderstanding the following operational description.

Messages from Room Controller to MLCT

ST: Start Talking message informs the MLCT that someone has started totalk in the ST is transmitted after a transition from a quiet room stateto a talker state is detected.

SP: Stop Talking message informs the MLCT that the room is no longer inthe active talker state.

STG: Start Graphics message is transmitted when any graphics-relatedSEND button is depressed.

SPG: Stop Graphics message is transmitted when the room has left thegraphics state.

The graphics state can be terminated by pressing any of the non-graphicscamera buttons (AUTO, CLOSE-UP, OVERVIEW).

PTSI: Push-to-see-room-I message is transmitted when an enabledpush-to-see button is pressed.

AVX: Automatic Video Switch message is transmitted when the room wantsto enter the automatic video switch mode.

Entering the automatic video switch mode is accomplished by pressing thecurrently lit push-to-see button.

GFUR: Generate Fast Update Request message is transmitted when theroom's TV Processor requests a fast update from the distant TVP. Thefast update request is conveyed to the room controller via the TVPinterface.

GR: Get Ready (for fast update) message is transmitted upon receipt of aGFUC message from the MLCT. This message is transmitted a predeterminedinterval prior to transmitting the fast picture update information bythe picture processor.

ARACK: Available Rooms Acknowledge message is transmitted as anacknowledgement that the AR message from the MLCT has been received. TheARACK message echoes the available room information bytes sent in the ARmessage.

Messages from MLCT to Room Controller

AVXACK: Automatic Video Switch Acknowledge message is transmitted inresponse to an AVX message received from a room controller.

STGACK: Start Graphics Acknowledge message is transmitted by the MLCT inresponse to an uncontested STG request.

STGNAK: Start Graphics Negative Acknowledge message is transmitted inresponse to an STG request when there is contention for rooms requestingto send graphics.

STGNAK is also transmitted to a room sending graphics to terminate theroom's graphics mode in the event of a network fault.

GFUC: Generate Fast Update Command message is transmitted when the MLCTis requesting the room TVP to send fast update video.

Upon receiving this message, the room controller will signal the TVprocessor via the TVP interface to initiate the fast update video and aGR message is transmitted a predetermined interval prior to transmissionof the fast video picture update.

GRI: Get ready (for fast update from room I) message is transmitted as apositive acknowledgement to either a GFUR message or a PTSI message thathas been previously transmitted from a room controller.

GRI may also be sent autonomously to a room before an unsolicited fastupdate video is being sent, such as when the new-talker room is changed.

AR: Available Rooms message is transmitted to inform the room controllerwhich push-to-see buttons are to be activated during the conference.

An AR message is transmitted to:

1. configure the conference at the start,

2. serve as a negative acknowledgement to a PTSI message that selects anunavailable room,

3. reconfigure the conference by adding or deleting rooms, or

4. declare assigned rooms to be active or inactive.

Messages from MLCT to MLCT

BR: Bus Request message is broadcasted to all the other MLCTs when theMLCT first assumes the bus-request state. (i.e. One or more of itsserving rooms is active, unmuted, and has talker.)

NBR: Non Bus Request message is broadcasted to all the other MLCTs whenthe MLCT first assumes the non-bus requested state. (i.e. All of itsserving rooms are either inactive, muted, or do not have a talker.)

NTI: New Talker Room I message is broadcasted once by an MLCT when itassigns the new talker to one of its serving rooms.

BL: Bus Release message is broadcasted once by the MLCT when it releasesthe new talker.

BRG: Bus Request Graphics message is broadcasted to all the other MLCTswhen the MLCT first assumes the graphics-bus-request state. (i.e. One ofits serving rooms is active and requesting to send graphics.)

NBRG: Non Bus Request Graphics message is broadcasted to all the otherMLCTs when the MLCT first assumes the non-graphics-bus-request state.(i.e. All of its serving rooms are either inactive or not requesting tosend graphics.)

NGI: New Graphics Room I message is broadcasted once by an MLCT when itassigns the new graphics to one of its serving rooms.

BLG: Bus Release Graphics message is broadcasted once by the MLCT whenit releases the new graphics.

GFURI: Generate Fast Update from Room I Request message is transmittedby the MLCT when it requires a video fast update from a room (room I)served by another MLCT.

GRI: Get Ready for fast update from room I message is transmitted overany broadcast link that is transmitting room I's video signal when roomI is sending a fast update.

PTSIJ: Room I is in push-to-see room J message is transmitted when aMLCT changes the mode of one of its serving rooms (room I). J containsdesired room id when room I is in the push-to-see mode and containsFF(hex) when room I is in the automatic switching mode.

Expanded Software Code

The video selection and fast update algorithms described below areperformed by control processor 1006 under control of the so-calledexpanded software code. Control processor 1006 expanded code operatesunder a real time multi-tasking operating system of a type known in theart. The expanded software code is arranged in tasks which are orderedaccording to their priority. While the control processor expanded codeis activated these tasks will always be in one of the following states:

DORMANT: The task is not scheduled to be run;

READY: The task is scheduled to be run;

RUNNING: The task is running (Only one task is running at a time);

"WAIT INTERRUPT": The task is waiting for a specified interrupt tooccur;

"WAIT POST": The task is waiting for a post from another task;

"WAIT TIMER": The task is waiting for a specified time period to elapse.

The control processor expanded mode code consists of a plurality ofhighly interactive tasks which can be functionally divided into thefollowing seven separate modules:

MS INTERFACE: Processes orders received from the microprocessor system(MS)101;

FACILITY ADMINISTRATOR: Monitors the facilities of the ports assigned inthe conference. Performs error recovery and operates the front panellights;

TRANSMISSION PATH INTERFACE: Performs the software interface with BIC1001, DEC 1002, BPC 1003, ENC 1004, and BOC 1005 hardware;

FACILITY ERROR MONITOR: Monitors facility errors and interrupts the MSwhen significant changes have occurred;

LEVEL 2 PROTOCOL: Handles the control channel communication over the DS1links connected to the rooms and other MLCTs in the conference;

MESSAGE PROCESSING: Processes messages received over the DS1 link'scontrol channels from rooms and other MLCTs by adjusting status andposting the proper tasks. Also, formats outgoing messages for the roomsand other MLCTs;

CONFERENCE CONTROL: Performs the video selection and fast updatealgorithms. This is done by interpreting the status changed by themessage processing and facility administrator modules and posting tasksin the message processing module to generate any required messages tothe rooms and other MLCTs.

MS Interface

The MS interface module 4710 (FIG. 47) processes the orders sent bymicroprocessor system (MS) 101 to bridge 103. These orders are:

1. Assign MLCT ID and speed for conference;

2. Assign port;

3. Adjust the MS status of a port;

4. Read status stored in bridge;

5. Download of encryption key;

6. Place the MLCT in the encrypted or clear state;

7. Clear bridge hardware alarms.

The first four orders are related to the administration of thefacilities used in the conference. When one of these orders arereceived, the MS interface module adjusts status and posts one of thefacility administrator modules' tasks.

The last three orders are related to operation of the BIC 1001, DEC1002, BPC 1003, ENC 1004, and BOC 1005 circuits. When one of theseorders are received the MS interface module adjusts status and posts oneof the transmission path interface module's tasks.

Transmission Path Interface

The transmission path interface module 4709 (FIG. 47) interfaces withthe BIC 1001, DEC 1002, BPC 1003, ENC 1004, and BOC 1005 circuits. Itperforms the communication required to:

1. Set up the audio summing matrixes;

2. Set up for encrypted audio or nonencrypted (clear) audio;

3. Monitor the circuits to determine hardware detected facility alarms.

The transmission path interface module sends an interrupt to the MS 101when it first determines a facility failure. Once one of the facilityalarms becomes active, the alarm remains active until the MS interfacemodule notifies the transmission path interface module of a MS requestto clear hardware alarms.

Facility Administrator

The facility administrator module 4708 (FIG. 47) maintains the basicstatus of the conference. It receives information from the followingmodules:

MS INTERFACE: relays information from the MS about the port assignmentsand MS status of the ports;

TRANSMISSION PATH INTERFACE: passes information about the facilityerrors detected by the bridge hardware;

LEVEL 2 PROTOCOL: informs facility administrator when control channelcommunication over each of the ports is connected or disconnected andwhen an error has occurred in the allocation of space for messages;

MESSAGE PROTOCOL: informs the facility administrator when the status ofother MLCTs or rooms served by other MLCTs change and when an error hasoccurred in the allocation of space for messages.

The facility administrator uses this information to maintain the MLCT'scondition (stand-alone or distributed) and each of its serving room'sactive and muted states. The MLCT is in the stand-alone condition if thelevel 2 protocol module indicates that at least one of the linksassigned as incoming from another MLCT or as a broadcast link has eitherthe MS status of the link being bad or the control channel communicationdisconnected.

While in the stand-alone condition the MLCT operates its own conferenceseparate from the other MLCTs. Otherwise, the MLCT is in the distributedcondition. A room is declared active, if it is assigned in theconference, the MS status of the port it is connected to is good, andthe level 2 protocol module indicates the control channel communicationwith the room is connected. Otherwise, the room is inactive. A room isdeclared muted (unmuted) if the transmission path module has muted(unmuted) the incoming audio signal of the port assigned to the room.

The facility administrator prompts the message processing module togenerate messages to the other MLCTs to keep them informed of the MLCT'sport assignments, MLCT's condition, and the active and muted states ofthe rooms served by the MLCT. Thus, if the MLCT is in the distributedcondition it is aware of the state of all rooms in the conference.

The facility administrator module is posted by the transmission pathinterface module when the muted state of any assigned link changes, bythe MS interface module when link assignments change, and by the messageprocessing module when changes occur to states (assigned, active ormuted) of rooms served by other MLCTs.

The facility administrator module processes all status to determine theactive and muted states of the rooms that the MLCT serves. If a changehas occurred to any of the assigned room's active states, the facilityadministrator module posts the NTP, NGP, and VSEL tasks. If a change hasoccurred to any of the assigned room's muted states, the facilityadministrator module posts the NTP task.

Level 2 Protocol Module

The level 2 protocol module performs the communication protocol of theDS-1 link's control channel. It transmits and receives messages from therooms connected to the MLCT via a BX.25 level 2 protocol. The BX.25level 2 protocol is described in the Bell System Technical Reference"Operations Systems Network Communications Protocol Specification,BX.25", Issue 3, June 1982 (Publication 54001). The level 2 protocolmodule uses a modification of the BX.25 level 2 protocol in itscommunication with the other MLCTs. This was necessary because of thebroadcast function of the inter-MLCT connections. This modified protocolwill be described in terms of the BX.25 level 2 protocol, definingparameters and deviations from BX.25.

The level 2 protocol module passes all messages received to the messageprocessing module. All messages to be transmitted are passed to thelevel 2 protocol module by the message processing module.

The level 2 protocol module notifies the facility administrator modulewhen control channels are first connected and when control channels aredisconnected due to facility failures. In the latter case the facilityerror monitor module is notified also.

The following BX.25 level 2 system parameters are defined:

"T1" is the time the transmitting station will wait for an acknowledgebefore retransmitting. In this example, T1 must be greater than theround trip satellite delay of 600 milliseconds.

"T3" is the maximum time allowed with no information messages beingtransmitted without polling the remote station.

"T4" is the time that a station will receive the idle channel (no flags)state before the station considers the link to be disconnected.

"N2" is the number of times that a message is transmitted andretransmitted.

"N1" is the number of bits in the information field and must contain anintegral number of octets (bytes). The largest information field allowedis 96 bits (12 bytes).

"R" is the maximum number (k) of sequentially numbered I frames that maybe outstanding (i.e. unacknowledged) at a given time and is defined inthis example to be 4.

The frame formats for MLCT-room communication protocol messages, i.e.,C-bit protocol, are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        FRAME FORMATS for MLCT-Room Messages                                          ______________________________________                                        Bit order of transmission                                                     12345678                                                                              12345678  12345678  16 to 1 12345678                                  Flag    Address   Control   FCS     Flag                                      F       A         C         FCS     F                                         01111110                                                                              8-bits    8-bits    16-bits 01111110                                  ______________________________________                                        Bit order of transmission                                                     12345678                                                                             12345678 12345678 123 . . . N                                                                           16 to 1                                                                             12345678                               Flag   Address  Control  Informa-                                                                              FCS   Flag                                                            tion                                                 F      A        C        I       FCS   F                                      01111110                                                                             8-bits   8-bits   N-bits  16-bits                                                                             01111110                               ______________________________________                                         FCS = frame checking sequence                                            

The BX.25 protocol defines two addresses, A=3 and B=1. The room, whichfunctions as the data circuit-terminating equipment (DCE), is assignedaddress A. The MLCT, which functions as the data terminal equipment(DTE), is assigned address B. The address is used to differentiatebetween commands and responses. When transmitting a command the roomuses the address of the MLCT, address B. When transmitting a responsethe room uses its own address, address A. Similarly, the MLCT usesaddress A (B) when transmitting a command (response).

The MLCT and room can exchange the following messages:

I: information

RR: receive ready

RNR: receive not ready

REJ: reject

DM: disconnected mode

SABM: set asynchronous balanced mode

UA: unnumbered message

FRMR: frame reject

DISC: Disconnect

Link connections between MLCTs and rooms are established according tothe BX.25 protocol. When no frames are being transmitted the MLCT androom will transmit continuous flags. The link connection is made by thefollowing sequence of events:

1. When the MLCT (room) detects the flags, it transmits a setasynchronous balanced mode (SABM) command with the poll bit set.

2. Upon receipt of the SABM command the room (MLCT) transmits anunnumbered acknowledged (UA) response with the final bit set and resetsboth its send and receive state variables, V(S) and V(R), to zero.

3. Upon receipt of the UA response, the MLCT (room) resets both its sendand receive state variables, V(S) and V(R), to zero.

Both the room and MLCT are now considered to be in the receive readystate.

The MLCT or room can disconnect the link by transmitting a DISC command.The room or MLCT will consider the link disconnected upon transmittingor receiving a DISC message.

While the link is connected the MLCT and room should never let thechannel go to the idle state. Interframe time fill (transmission ofcontiguous flags between frames) should be used to keep the channelactive. If the MLCT or room detects an idle channel, they consider thelink disconnected.

In an MLCT-MLCT connection, each MLCT will receive signals from allother MLCTs. An MLCT may desire to send a message to one MLCT in a MLCTconference and all the other MLCTs must not respond to that message. Inother cases an MLCT may want to send a message to all MLCT'S in aconference. Thus, the receive section of each MLCT has to be capable ofdisregarding any messages not intended for it and accepting only thosethat are to be processed by that MLCT.

Herein the term "logical link" refers to the communication protocolestablished over one of the MLCT's broadcast links. The logicalMLCT-MLCT link is a one-way information transfer link. In a multipleMLCT conference each MLCT will have at least one logical MLCT link(incoming link from another MLCT) and at least one logical broadcastlink (outgoing link to other MLCTs). The MLCT maintains a send statevariable, V(S), for each logical broadcast link, but no receive statevariable. Similarly, the MLCT maintains a receive state variable, V(R),for each logical MLCT link, but no send state variable.

The frame formats for MLCT-MLCT messages are shown in Table 6. TheMLCT-MLCT frame contains two addresses, the destination address (DADDR)and the source address (SADDR).

                                      TABLE 6                                     __________________________________________________________________________    FRAME FORMATS for MLCT-MLCT Messages                                          __________________________________________________________________________    Bit order of transmission                                                     12345678                                                                             12345678                                                                             12345678                                                                            12345678                                                                              16 to 1                                                                             12345678                                    Flag   DADDR  SADDR Control FCS   Flag                                        F      A      A     C       FCS   F                                           01111110                                                                             8-bits 8-bits                                                                              8-bits  16-bits                                                                             01111110                                    __________________________________________________________________________    Bit order of transmission                                                     12345678                                                                            12345678                                                                           12345678                                                                            12345678                                                                           123 . . . N                                                                         16 to 1                                                                            12345678                                     Flag  DADDR                                                                              SADDR Control                                                                            Infor-                                                                              FCS  Flag                                                               mation                                                  F     A    A     C    I     FCS  F                                            01111110                                                                            8-bits                                                                             8-bits                                                                              8-bits                                                                             N-bits                                                                              16-bit                                                                             01111110                                     __________________________________________________________________________     DADDR = destination address                                                   SADDR = source address                                                        FCS = frame checking sequence                                            

The BX.25 protocol defines only two addresses, A=3 and B=1, that areused to delineate between commands and responses. These two addressesare not sufficient to handle the MLCT-MLCT messages. The MLCT cantransmit a message over a broadcast link and receive the acknowledgeover any MLCT link (which could be assigned to any of the MLCT's sixports).

Table 7 shows how the MLCTs will assign their source addresses forbroadcast links. Bit 8 is used to distinguish MLCT addresses from roomaddresses (Bit 8=1 indicates an MLCT, bit 8=0 indicates a room). Bit 7indicates whether the frame is a command (bit 7=0) or a response (bit7=1).

                  TABLE 7                                                         ______________________________________                                        MLCT-MLCT SOURCE ADDRESS FORMAT                                               ______________________________________                                        Bit       8         7       6 5 4  3 2 1                                      MLCT                                                                          Individual                                                                              Always    C/R     Port ID                                                                              MLCT ID                                    Address   One               (0-5)  (0-5)                                      ______________________________________                                         C/R = for command frame, 1 for response frame                                 MLCT ID = ID of MLCT transmitting the frame                                   Link ID = ID of port over which MLCT is transmitting the frame           

MLCTs store their MLCT ID in bits 1-3 and the port ID of the output linkover which the message is being transmitted in bits 4-6. Whentransmitting a response to a command, MLCTs always use the commandframe's source address as the destination address in the response frame(with bit 7=0). Thus, the destination address of a response framespecifies not only the MLCT, but the output link (broadcast link) towhich the response is directed. The port ID is required, since theresponses to commands transmitted over a broadcast link can be receivedover any of the MLCT's input links (that is assigned as a MLCT link).

Table 8 shows how the MLCTs will assign their destination addresses forbroadcast links. As in MLCT-MLCT source addresses bit 8 distinguishesMLCT addresses from room addresses. Bit 7 indicates whether thedestination address is an individual address (bit 7=0) or a broadcastaddress (bit 7=1).

                  TABLE 8                                                         ______________________________________                                        MLCT-MLCT DESTINATION ADDRESS FORMATS                                         ______________________________________                                        Bit       8         7       6 5 4  3 2 1                                      MLCT                                                                          Individual                                                                              Always    Always  Port ID                                                                              MLCT ID                                    Address   One       Zero    (0-5)  (0-5)                                      ______________________________________                                        Bit     8      7      6    5    4    3    2    1                              MLCT                                                                          Broadcast                                                                             One    One    One  One  One  One  One  One                            Address                                                                       ______________________________________                                         M(0-5) indicates message is addressed to MLCT (0-5)                      

The MLCT-MLCT individual destination address format is the same as thatof the MLCT-MLCT source address (with bit 7=1). This comes from the factthat MLCTs use the source address of frames received over a MLCT link todirect responses to that particular MLCT link. In the MLCT-MLCTindividual destination address, the MLCT ID is the ID of the MLCT thatis to receive the frame and the port ID is the ID of the particular link(broadcast link) in that MLCT to which the frame is directed.

MLCTs use the MLCT-MLCT broadcast address as the destination addresswhen broadcasting commands to the other MLCTs. If a command frame with aMLCT-MLCT broadcast address requires a response, the transmitting MLCTexpects a response from all other MLCTs in the conference.

The logical links and logical MLCT link can exchange all of thefollowing messages: RR, RNR, REJ, SABM, DISC, DM, UA, FRAMR.

    ______________________________________                                        Broadcast Link   MLCT Link                                                    ______________________________________                                        I      information   RR      receive ready                                    RNR    receive not ready                                                                           RNR     receive not ready                                DISC   disconnect    REJ     reject                                           SABM   set asynchronous                                                                            DM      disconnected mode                                       balanced mode UA      unnumbered message                                                    FRMR    frame reject                                     ______________________________________                                    

The procedure in establishing logical link connections was derivedassuming one basic premise: The inability to communicate over onebroadcast link in the conference stops the exchange of informationbetween MLCTs. Losing an MLCT link or broadcast link causes the MLCT togo to the stand-alone condition. MLCTs in the stand-alone condition donot transmit information frames.

The rules for establishing and maintaining logical link connections areas follows:

1. When not transmitting frames over a broadcast link, an MLCT willtransmit continuous flags. (i.e. The connected, but idle state is notallowed in MLCT-MLCT communication.)

2. The MLCT transmits a set asynchronous balanced mode (SABM) commandwith the poll bit set.

3. Upon receiving the SABM command over an MLCT link each of the otherMLCTs will transmit an unnumbered acknowledged (UA) response with thefinal bit set over any of their broadcast links and reset their receivestate variable, V(R), for that MLCT link. The MLCTs that transmitted aUA response consider that logical MLCT link connected at this point.

4. Upon receiving a UA response (with the final bit set) from each ofthe other MLCTs the first MLCT resets the send state variable, V(S), forthe logical broadcast link. The first MLCT considers that logicalbroadcast link connected at this point.

After all of the MLCTs perform the above steps for each of theirbroadcast links, full communication between the MLCTs is established.

A MLCT considers an established logical MLCT link to be disconnected ifit fails to detect flags for more than time period T4 or it receives aDISC command over the link. A MLCT considers a logical broadcast link tobe disconnected, if it transmits and retransmits a frame N2 timeswithout receiving the required response from all of the other MLCTs.

The MLCTs exchange the MLCT messages according to the proceduredescribed in Publication 54001 noted above. The only difference is thatwhen the MLCT transmits a command with a MLCT-MLCT broadcast address theMLCT must receive a response from each of the other MLCTs. If it failsto receive a response from any of the other MLCTs, the MLCT willretransmit the command. The command is transmitted and retransmitted upto N2 times. If after N2 times the MLCT still fails to receive all ofthe required responses, it disconnects the logical broadcast link. Theother MLCTs that have already transmitted a response acknowledgingreceipt of the command will receive the command again and:

1. Ignore the command, if it is an information frame but send theresponse again.

They will recognize the retransmission because the send sequence number,N(S), will be out of sequence.

2. Execute the command and send the response again, if it is asupervisory or unnumbered command.

The MLCT will never send a DM response since it always wants the linkconnected and active.

Included in the information messages are the VTS operating code. The VTSoperating code, hereinafter opcode, is defined as the first byte (8binary bits) contained in the information field of a BX.25 messageexchanged between rooms and MLCTs in a conference. There are four basictypes of VTS opcodes; room to MLCT opcodes, MLCT to room opcodes, MLCTto MLCT opcodes, and room to room opcodes. The bit pattern of the VTSopcodes was chosen to allow easy recognition of the type of opcode. Inaddition, the opcodes which involve the MLCT have been assignedcontiguously to simplify the MLCT's programming.

Bits 7 and 8 are assigned as follows:

    ______________________________________                                        bit     8       7                                                             ______________________________________                                        0           0       MLCT to MLCT opcode                                       0           1       MLCT to room opcode                                       1           0       Room to MLCT opcode                                       1           1       Room to room opcode                                       ______________________________________                                    

Thus, bit 8 indicates who the message is from and bit 7 indicates towhom the message is destined (0--MLCT, 1--room).

    ______________________________________                                        MLCT to MLCT OPCODE FORMATS                                                   Information Field                                                             Message Byte 1   Byte 2      Byte 3                                           ______________________________________                                        NBR     00       --          --                                               BR      01       --          --                                               NBRG    02       --          --                                               BRG     03       --          --                                               BL      04       --          --                                               NTI     05       room ID     --                                               BLG     06       --          --                                               NGI     07       room ID     --                                               PTSIJ   08       from room ID                                                                              desired room ID                                  GRI     09       room ID                                                      GFURI   0A       room ID                                                      LNKST   0B       ded room ID                                                  MSTAT   0C       **see below for additional bytes                             ______________________________________                                         **The additional bytes for the MSTAT opcode are:                              byte 2: bits 1-3 contain the MLCT's ID                                        bits 4-6 MLCT mode (SC=010, VC=011)                                           bit 7 MLCT encryption state (0=clear, 1=encrypted)                            bit 8 MLCT condition (SA=0, DS=1)                                             byte 3: bits 1-6 are assigned MLCT bits (one bit for each MLCT).              Bit set indicates the room is assigned.                                       Bit 1-6 corresponds to MLCT 1-6.                                              bits 7,8 are always zero                                                      byte 4 zero                                                                   byte 5 assigned room bits (one bit for each room).                            Bit set indicates the room is assigned.                                       Bit 1-6 corresponds to room 1-6 respectively.                                 byte 6 zero                                                                   byte 7: active room bits (one bit for each room).                             Bit set indicates the room is active.                                         Bit 1-6 corresponds to room 1-6 respectively.                                 byte 8 zero                                                                   byte 9 unmuted room bits (one bit for each room).                             Bit set indicates the room is unmuted.                                        Bit 1-6 corresponds to room 1-6 respectively.                            

MLCT to Room opcode formats

    ______________________________________                                        Information Field                                                             Message    Byte 1  Byte 2   Byte 3                                            ______________________________________                                        AVXACK     00      --       --                                                AR         01      room ID  *available rooms                                  STGNAK     02      --       --                                                STGACK     03      --       --                                                GFUC       04      --       --                                                GRI        05      room ID  --                                                ______________________________________                                         *bit 0-5 of available rooms byte is set if room 0-5 respectively is           available for pushto-see.                                                     (bits 6 and 7 are always zero)                                           

Room to MLCT opcode formats

    ______________________________________                                        Information Field                                                             Message    Byte 1  Byte 2   Byte 3                                            ______________________________________                                        SP         00      --       --                                                ST         01      --       --                                                SPG        02      --       --                                                STG        03      --       --                                                ARACK      04      room ID  *available rooms                                  PTSi       05      room ID  --                                                AUX        06      --       --                                                GFUR       07      --       --                                                GR         08      --       --                                                ______________________________________                                         *available rooms byte should be same as the available rooms byte in the       last AR message received from the MLCT.                                  

Message Processing Module

The message processing module 4707 (FIG. 47) is the interface betweenthe conference control module and level 2 protocol module. The messageprocessing module interprets the messages received from rooms and otherMLCTs, adjusts the related status, and posts the proper task in theconference control module. The message processing module allows theconference control module to be nearly unaware of how rooms and MLCTsare connected to this MLCT.

When the message processing module processes a message received from aroom or another MLCT it adjusts status and post tasks according to thetype of message. The processing of the various messages is as follows:

    ______________________________________                                        Message                                                                       Received  Action Taken                                                        ______________________________________                                        ST/SP     The room that transmitted the ST (SP)                                         message is marked as having (not having) a                                    talker and the NTP task is posted.                                  BR/NBR    the MLCT that transmitted the BR (NBR)                                        message is marked as having (not having) a                                    talker and the NTP task is posted.                                  NTI/BL    The NTI (BL) message is stored in the last                                    NTI (BL) message received variable, the NTI                                   (BL) message received flag is set, and the                                    NTP task is posted.                                                 STG/SPG   The graphics request flag of the room that                                    transmitted the STG (SPG) message is set                                      (cleared) and the NGP task is posted                                BRG/NBRG  The MLCT that transmitted the BRG (NBRG)                                      message is marked as having (not having)                                      a graphics request and the NGP task                                           is posted.                                                          NGI/BLG   The NGI (BLG) message is stored in the last                                   NGI (BLG) message received variable, the NGI                                  (BLG) message received flag is set, and the                                   NGP task is posted.                                                 PTSI/AVX  The room that transmitted the PTSI (AVX)                                      message is marked being in the PTSI (AVX)                                     mode and the VSEL task is posted.                                   GFUR/GFURI                                                                              Room I's request fast update received flag                                    is set and the FUGEN task is posted when                                      the GFUR message is received from a room                                      receiving room I's video or a GRURI message                                   is received from another MLCT.                                      GR/GRI    Room I's fast update received flag is set and                                 the VSWCH task is posted when a GR message                                    is received from room I or a GRI message is                                   received from another MLCT.                                         LNKST     The link's dedicated room status is adjusted                                  according to the LNKST message and the                                        FADMIN task is posted.                                              MSTAT     The status of the rooms (assigned, active,                                    and muted states) is adjusted according to                                    the MSTAT message and the FADMIN task is                                      posted.                                                             ______________________________________                                    

Conference Control Module

The conference control module performs the video selection and fastupdate algorithms. It monitors the state of each room in the conferenceto determine the desired room to be viewed for each room. If some roomsare not viewing their desired room the conference control moduleinitiates fast updates and performs a video switch for the rooms whenthe fast updates arrive.

Fast updates are also initiated upon request from rooms who have lostsynchronization with their incoming video signal. The conference controlmodule maintains timing windows for each room to ensure that fast updatsand video switches are performed properly.

Facility Error Monitor Module

The facility error monitor module sets error flags when various facilityerrors occur. Once an error flag is set it will remain set until aspecified hold interval elapses without its respective error occurringagain. The facility error monitor module interrupts the MS when any ofthese error flags transition. The hold interval allows the MS to noticetransient errors that the bridge may detect and recover from before theMS has a chance to read the status. The facility error monitor modulereceives input from the facility administrator, level 2 protocol,message processing, and conference control modules.

Video Selection and Fast Update Algorithms-General

The tasks that perform the video selection algorithm and the fast updatealgorithm, in accordance with the aspects of the invention, arepresented in FIG. 47. The video selection algorithm is implemented inthe New Talker Processing (NTP) task 4701, New Graphics Processing (NGP)task 4702, and Video Select (VSEL) task 4703. The fast update algorithmis implemented in the Fast Update Generator (FUGEN) task 4704, FastUpdate Monitor (FUMON) task 4705, and Video Switch (VSWCH) task 4706.

Message Processing module 4707, Facility Administrator module 4708,Transmission Path Interface module 4708 and MS Interface module 4710 alldescribed above administer operation of the video selection and fastupdate algorithms.

The tasks communicate with each other by setting flags and sending apost to another task via an operating system of a type well known in theart in control processor 1006. The usual manner is to have anoriginating task store any required information and set a flag whichsignals the destination task to perform a certain operation. Theoriginating task sends a post to the destination task. When thedestination task runs, it checks the flag to see if the operation needsto be performed. If the flag is set the destination task clears the flagand performs the operation. Most of the status variables which are notcontrolled by one task are flags.

Video Selection Algorithm

The video selection algorithm allows, in accordance with an aspect ofthe invention, two modes of conferences, namely, a "Sender's Choice"conference and a "Viewer's Choice" conference. In a Sender's Choiceconference all rooms stay in the automatic video switching (AVX) mode.The video switching decisions for rooms in the AVX mode are performedautomatically by the MLCT.

During a conference, an MLCT maintains active and muted status of itsserving rooms. A room is active if no facility errors, other than errorsin the audio signal, are detected in the port connected to the room.That is to say, good control channel communication is established.Facility errors that affect the room's active status are loss of theBX.25 communication protocol with the room and port errors detected bythe network. If the MLCT detects problems in decrypting the room's audiothe MLCT declares the room muted. That is to say, the room is muted ifunsatisfactory audio information is detected. The MLCT exchanges theactive and muted status of its serving rooms with all the other MLCTs inthe conference via the MLCT status (MSTAT) message. MLCTs use the room'sactive and muted status to determine how the room may function in theconference.

At the start of a conference, each MLCT transmits an available room list(AR) message to its serving rooms. The available room list specifieswhat rooms can be selected for viewing. In a Viewer's Choice conference,the available room list is the list of active rooms. When a room'sactive/inactive status changes, each MLCT sends an updated availableroom list (i.e. an AR message) to each of its serving rooms. In aSender's Choice conference, the available room list is always empty.

The conference rooms have certain so-called activity states. The term"graphics bus contention" is used in the following discussion. Graphicsbus contention exists when a room requests to send graphics and anotherroom is either already assigned the new-graphics or is requesting tosend graphics. The existence of graphics bus contention forces the MLCTto reject all requests for sending graphics at that time. If thenew-graphics is assigned to a room when graphics bus contention occurs,that room remains the new-graphics.

During a conference, an MLCT maintains a new-graphics state and anew-talker state for itself and a single-room-activity state for each ofits serving rooms.

Each room served by an MLCT will be in one of the followingsingle-room-activity states:

1. Idle state--the room does not have a talker and is not sendinggraphics.

2. Talker-only state--the room has a talker, but is not sendinggraphics.

3. Graphics-only state--the room is sending graphics, but does not havea talker.

4. Graphics-and-talker state--the room has a talker and is sendinggraphics.

When a room first becomes active, its MLCT places the room in the idlestate. The room remains in the idle state until the MLCT receives astart-talking (ST) or a start-graphics-request (STG) message from theroom. If a room in the idle state transmits ST, the MLCT places the roomin the talker-only state. If a room in the idle state transmits an STGmessage and graphics bus contention exists, the MLCT rejects the requestby transmitting a start-graphics-negative-acknowledge (STGNAK) messageand keeps the room in the idle state. Otherwise, the MLCT assigns thenew-graphics to the room, places the room in the graphics-only state andsends the room a start-graphics-acknowledge (STGACK) message.

A room in the talker-only state remains in that state until the MLCTreceives from the room either a stop-talking (SP) message or an STGmessage with no graphics bus contention present. If the room sends SP,the MLCT places the room in the idle state. If the room sends STG whengraphics bus contention exists, the MLCT sends STGNAK to the room andthe room remains in the talker-only state. Otherwise, the MLCT assignsthe new-graphics to the room, places the room in the graphics-and-talkerstate, and sends STGACK to the room.

A room in the graphics-only state remains in that state until the MLCTreceives a stop-graphics (SPG) message or an ST message from the room.In the former case, the MLCT places the room in the idle state. In thelatter, the MLCT places the room in the graphics-and-talker state.

A room in the graphics-and-talker state remains in that state until theMLCT receives an SP message or an SPG message from the room. In theformer case, the MLCT places the room in the graphics-only state. In thelatter, the MLCT places the room in the talker-only state.

The facility administrator module maintains the active and muted statusfor each of the rooms served by the MLCT. When the facilityadministrator module adjusts a room's active or muted status it poststhe NTP, NGP and VSEL tasks to update their processing.

A MLCT determines the video to transmit to each of its serving(assigned) rooms as follows:

1. To view the selected room--if the room is in the PTS mode or entersthe PTS mode, the room receives the video from the room it specified inthe last PTS request.

2. To view the new-graphics--If the room is in the AVX mode and ifanother room, room I, is currently assigned the new graphics, the roomreceives the video from room I.

3. To view the new talker--If the room is in the AVX mode and if anotherroom, room I, is currently assigned the new talker, the room receivesthe from room I.

4. To view the last new-talker--If the room is in the AVX mode and ifanother room, room I, was last assigned the new talker, the roomreceives the video from the last NTR.

5. To view the second to last new-talker--If the room is in the AVX modeand if another room, room I, was the second to last room assigned thenew talker, the room receives the video from room I.

6. If none of the above conditions apply, the room continues to receivethe video it is presently receiving.

For each broadcast link that is dedicated to a room served by this MLCTthe MLCT transmits the video signal received from that room. For eachbroadcast link that is dedicated to a room served by another MLCT theMLCT determines the video to transmit as follows. If the room is in thepush-to-see mode specifying a room served by this MLCT, the MLCTtransmits the video of that specified room. Otherwise, the MLCT selectsthe video signal to transmit according to the priorities established forundedicated broadcast links. The MLCT video signal selection prioritiesfor its undedicated broadcast links are:

1. Transmit graphics room's output video if the new the new graphics isassigned to one of the MLCT's serving rooms.

2. Transmit the new talker room's output video if the new talker isassigned to one of the MLCT's serving rooms.

3. If the last room, room I, that was assigned the new talker is servedby this MLCT, then transmit room I's output video.

4. If the second to last room, room I, assigned the new talker is servedby this MLCT, then transmit room I's output video.

5. If none of the above cases apply leave the video transmission as itis.

Following these priorities the other MLCTs will always have the videosignals required to ensure each of their serving rooms receive theirdesired video.

New Talker Processing (NTP) Task

New talker processing (NTP) task 4701 resolves contention amongconference locations for the video to be transmitted. To this end, itmaintains a talker bus state which is a summary of the talkers in theMLCT's serving rooms. If none of the MLCTs serving rooms have a talker(i.e. all of the serving rooms are in the graphics only or idle state)the MLCT is in the non-bus-request state. If at least one of the servingrooms has a talker (is in the talker-only or graphics-and-talker state),then the MLCT is in the bus-request state. When a MLCT transitions tothe bus-request (non-bus-request) state it broadcasts a bus-request, BR,(non-bus-request, NBR) message to the other MLCTs. Thus, the NTP taskknows the talker bus state of all MLCTs in the conference.

The NTP task uses the MLCT's serving room's single room activity statesand the other MLCTs talker bus state to maintain a new-talker (NT)states. A MLCT is in the NT-idle state if the new talker is not assignedand none of its serving rooms has a talker (i.e., all rooms are in theidle or graphics-only state). At the start of the conference, all MLCTsare in the NT-idle state. If a MLCT in the NT-idle state detects atalker in one of its serving room's (i.e., its talker bus is in thebus-request state), then the MLCT assumes the bus-contend state. A MLCTin the bus-contend state keeps checking to see if one of its servingrooms has an uncontested talker. That is to say, no change in the videobeing transmitted to the rooms is made until one and only one room has atalker during a predetermined talker contention timing interval. TheMLCT remains in the NT-bus-contend state until one of the followingevents occurs:

1. All of the rooms served by the MLCT stop talking. (All of the roomsserved by the MLCT are in either the idle state or the graphics-onlystate.) In this case, the MLCT enters the NT-idle state.

2. The NTP task determines that one of the MLCT's serving rooms has anuncontested talker. When only one of the MLCT's serving rooms has atalker (i.e. only one serving room is in talker-only orgraphics-and-talker state) and all of the other MLCTs talker buses arein the non-bus-request state, the MLCT starts a timing window. If thetiming window expires without detecting any other talkers (i.e. anyother room served by the MLCT assuming the talker-only orgraphics-and-talker state and none of the other MLCT's talker busesassuming the bus-request state), the MLCT assumes the serving room hasan uncontested talker. In this case, the NTP task assigns the new-talkerto that room (room I), transmits a NTI message to the other MLCTsspecifying that room I is the new-talker and puts the MLCT in theNT-this-MLCT state.

3. A room served by another MLCT is assigned the new-talker. (The MLCTreceives an NTJ message from another MLCT.) In this case, the NTP taskassigns the new-talker to room J and puts the MLCT in the NT-other-MLCTstate.

4. The MLCT receives a bus-release (BL) message from another MLCT. (Thisis an invalid condition.) In this case, the NTP task would put the MLCTin the NT-idle state. If one of the rooms served by the MLCT still has atalker (the room is in the talker-only state or graphics-and-talkerstate), the MLCT would immediately return to the NT-bus-contend state.

A MLCT can only enter the NT-this-MLCT state from the NT-bus-contendstate. Once in the NT-this-MLCT state, the MLCT remains there until oneof the following events occurs:

1. The room that is assigned the new-talker stops talking. (The roomassumes the idle state or graphics-only state.) In this event, the NTPtask transmits BL and puts the MLCT in the NT-idle state.

2. The MLCT receives a BL message from another MLCT. (This is an invalidcondition, since only the MLCT serving the new-talker should transmitthe BL message.) In this event, the NTP task would put the MLCT in theNT-idle state.

3. The MLCT receives a NTI message from another MLCT. (This is aninvalid condition, since no MLCT should transmit a NTI message when thenew-talker exists.) In this event, the NTP task would transmit a BLmessage to all of the other MLCTs to force them into the NT-idle stateand put the MLCT in the NT-idle state.

A MLCT enters the NT-other-MLCT state when it is in the NT-idle orNT-bus-contend state and receives a NTI message. The MLCT remains in theNT-other-MLCT state until it receives a BL message from another MLCT.When this occurs, the MLCT enters the NT-idle state.

If the MLCT should receive a NTJ message while in the NT-other-MLCTstate (this is an invalid condition), the MLCT would remain in theNT-other-MLCT state and assume room J to be the new-talker.

If the MLCT is in the stand-alone condition then the NTP task does notcheck the other MLCTs talker bus states. It instead performs theassignment of the new-talker as if this MLCT is the only MLCT in thecall.

The timing interval window used to resolve talker contention shouldprevent more than one MLCT from assigning the new talker at a time. If aMLCT has assigned the new talker (is in the NT-this-MLCT state) andreceives a NTI message from another MLCT, the first MLCT transmits a BLmessage placing all MLCTs back into the NT-idle state.

FIGS. 48, 49 and 50 connected as shown form a flowchart of the stepsperformed in the new talker process task (NTP) by control processor 1006of FIG. 39. The program is stored in memory 3904 (FIG. 39).

Briefly, the NTP task maintains the new talker state (NT) and talker busstate. It processes changes to the rooms served by the MLCT, i. e.,active, unmuted or talker and to the new talker and talker bus states ofother MLCTs in the conference. Accordingly, it updates the new talkerstate. Additionally, it also updates the last new talker states andsecond to last new talker states.

The NTP task is entered via oval 4801. Thereafter, operational block4802 initializes the variables controlled by the NTP task, namely, newtalker=bus-release, talker bus=nonbus request, talkerid=unassigned,timecount=4, transmits bus release (BL) message, transmits bus request(BR) message, and waits for post.

Operational block 4803 updates the talker bus state (TBUS) of the MLCTand transmits a BR/NBR message if a change in the talker bus stateoccurs.

Conditional Branch point 4804 tests the new talker (NT) state todetermine if NT is assigned. If the test result is true, i.e., yes, NTis assigned and control is transferred to conditional branch point 4805.However, if the test in step 4804 is false, i.e., no, NT is not assignedand control is transferred to conditional branch point 4806. Conditionalbranch point 4806 tests to determine whether this MLCT is in a standalone condition and whether a new talker I (NTI) message has beenreceived. If the test result is false, i.e., a NTI message is notreceived or the MLCT is in the stand alone condition, control istransferred to operational block 4807 which calls sub-routine CONTEND. Aflow chart of sub-routine CONTEND is shown in FIG. 51 and describedbelow. Upon completion of the CONTEND sub-routine control is returned toblock 4803.

If the test result in step 4806 is true, control is transferred toconditional branch point 4808 which tests to determine if the NTImessage has assigned the new talker to a room served by this MLCT. Ifthe test result in step 4808 is true, control is transferred tooperational block 4809 and the bus release message (BL) is transmitted.Thereafter, the CONTEND sub-routine is called via operational block4810. Upon completion of the CONTEND sub-routine control is returned tooperational block 4803. If the test in step 4808 is false, control istransferred to operational block 4811 and the new talker is assigned tothe room specified by the new talker (NTI) message. Then, it posts theVSEL task, talkid is set to equal unassign and time count is set to 4.

Operational block 4812 causes a 5-second wait interval and operationalblock 4813 causes a wait-for-post to occur. Thereafter, control isreturned to operational block 4803.

Returning to conditional branch point 4805, a test is made to determineif the new talker (NT) room is served by this MLCT. If the test resultis false, control is transferred to conditional branch point 4814.

If the test result in step 4805 is true, control is transferred toconditional branch point 4815 and a test is made to determine if theMLCT is in a stand-alone-condition. If the test result in step 4815 istrue, control is transferred to conditional branch point 4816. If thetest result is false, control is transferred to conditional branch point4817 and a test is made to see if a bus release message is received. Ifnot, control is transferred to conditional branch point 4818. If thetest result in step 4817 is true, control is transferred to operationalblock 4819. Operational block 4819 calls the NTSAVE sub-routine, theflowchart of which is shown in FIG. 52 and is described below.Thereafter, control is transferred to operational block 4820 and theCONTEND subroutine noted above is called. Then, control is returned tooperational block 4803.

Returning to conditional branch point 4818, a test is made to determineif the new talker message has been received. If not, control istransferred to conditional branch point 4816. If the test result istrue, control is transferred to operational block 4821.

Operational block 4821 causes a bus release (BL) message and a busrequest (BR) message to be transmitted to the other MLCTs in theconference. Thereafter, control is transferred to operational block4819.

Conditional branch point 4816 tests to see if the new talker room isactive, unmuted and talking. If true, control is transferred tooperational block 4822 and a wait-for-post is caused to occur.Thereafter, control is returned to operational block 4803.

Returning to conditional branch point 4816, if the test result is false,control is transferred to operational block 4823 and a bus release (BL)message is transmitted to the other MLCTs. Thereafter, control istransferred to operational block 4819.

Conditional branch point 4814 tests to see if this MLCT is in astand-alone-condition. If true, control is transferred to operationalblock 4824. If false, control is transferred to conditional branch point4825. Conditional branch point 4825 tests to determine if a bus release(BL) message has been received. If the result is false, control istransferred to conditional branch point 4826. However, if the result istrue, control is transferred to operational block 4827 and the NTSAVEsub-routine is called. Thereafter, operational block 4828 causes theCONTEND sub-routine noted above to be called. Then, control is returnedto operational block 4803. Operational block 4824 unassigns the newtalker, last new talker and second to last new talker. Thereafter,control is transferred to operational block 4829 and the CONTENDsub-routine noted above is called. Thereafter, control is transferred tooperational block 4803.

Returning to conditional branch point 4826, a test is made to see if aNTI message has been received. If the test is false, control istransferred to operational block 4830 which causes a wait-for-post.Thereafter, control is returned to operational block 4803. If the testresult is true, i.e., NTI message has been received, control istransferred to operational block 4831 and the NTSAVE sub-routine notedabove is called. Thereafter, control is transferred to conditionalbranch point 4832.

Conditional branch point 4832 tests to see if the new talker (NTI)message assigns the new talker to a room served by this MLCT. If thetest result is false, control is transferred to operational block 4833which assigns the new talker to the room specified by the NTI message.Thereafter, control is transferred to operational block 4834 which poststhe VSEL task noted above. Then, operational block 4835 causes await-for-post. Thereafter, control is transferred to operational block4803. If the test result in step 4832 is true, i.e., a new talker isassigned to a room served by this MLCT, control is transferred tooperational block 4836 and a bus release (BL) message is transmitted.

Then, control is transferred to operational block 4837 and the CONTENDsub-routine noted above is called. Thereafter, control is returned tooperational block 4803.

FIG. 51 shows a flowchart of the steps performed in the CONTENDsub-routine utilized in the new talker processing task described inrelationship to FIGS. 48, 49 and 50 by control processor 1006 (FIG. 39).This program is also stored in memory 3904 (FIG. 39).

The CONTEND sub-routine is entered via oval 5101. Thereafter,conditional branch point 5102 tests to determine if the MLCT has theonly active and unmuted serving room that is talking. If the test resultis false, i.e., there is either none or more than one room contendingfor the new talker state and control is transferred to operational block5103.

If the test result in step 5102 is true, i.e., there is only one servingroom having a talker, control is transferred to conditional branch point5104.

Conditional branch point 5104 tests to determine if the MLCT is not inthe stand-alone-condition and another MLCT has its talker bus=to busrequest. If the test result is true, control is transferred tooperational block 5103. However, if the test is false, control istransferred to conditional branch point 5105.

Conditional branch point 5105 tests to determine if the talkid equalsthe id of the talker room. If the test is false, control is transferredto operational block 5106 which sets talkid equal to the id of thetalking room and sets time count=4. Then control is transferred tooperational block 5107. If the test in step 5105 is true, control istransferred directly to operational block 5107 and time count is set totime count=time count -1.

Conditional branch point 5108 tests to determine if time count=0. If thetest is true, 4 passes have been made through the new NTP task andcontrol is transferred to operational block 5109. However, if the testresult is false, control is transferred to operational block 5110 whichcauses a wait-for-200 milliseconds. Then, control is transferred to oval5111 and returns to the NTP task.

Operational block 5109 assigns the new talker to the room specified bytalkid, transmits a NTI message to the other MLCTs in the conference,posts the VSEL task, and waits 5 seconds. Then, control is transferredto operational block 5103 where talkid is set to unassign, time count isset equal to 4 and operational block 5112 causes a wait-for-post.Thereafter, control is transferred to oval 511 andl control is returnedto the NTP task.

FIG. 52 shows a flowchart of the steps performed by processor 1006 ofFIG. 39 in the NTSAVE sub-routine utilized in the new talker processortask shown in FIGS. 48, 49 and 50. The NTSAVE program is also stored inmemory 3904 (FIG. 39).

The NTSAVE sub-routine is entered via oval 5201. Thereafter, conditionalbranch point 5202 tests to determine if the last new talker is the newtalker. If the test is false, control is transferred to operationalblock 5203 which sets the second to last new talker to the last newtalker and sets the last new talker equal to the new talker. Control isthen transferred to operational block 5204. If the test in conditionalbranch point 5202 is true, control is transferred directly tooperational block 5104 which sets the new talker equal to bus release.Thereafter, control is returned via oval 5205 to the new talkerprocessing task shown in FIGS. 48, 49 and 50.

New Graphics Processing (NGP) Task

New Graphics Processing (NGP) task 4702 resolves contention among theconference locations for the graphics to be transmitted. To this end, itmaintains a graphics bus state which is a summary of the requests forgraphics by the MLCT's serving rooms. It should be noted that there isan interdependence between the MLCT's new-graphics state and all of thesingle-room-activity states. If the MLCT is in the NG-this-MLCT state,then one and only one of its serving rooms is in the graphics-only stateor graphics-and-talker state. That is, one of the rooms served by theMLCT is assigned the new-graphics. If the MLCTs is in the NG-idle stateor NG-other-MLCT state, then all its serving rooms are in the idle stateor talker-only state. That is, none of the rooms served by the MLCT isassigned the new-graphics.

A serving room is considered to be requesting graphics if it is activeand has transmitted an STG message to the MLCT and the MLCT has nottransmitted a subsequent STGNAK message. If none of the MLCTs servingrooms are requesting graphics the MLCT is in thenon-graphics-bus-request state. If at least one of the MLCT's servingrooms is requesting graphics, the MLCT is in the graphics-bus-requeststate. When an MLCT transitions to the graphics-bus-request(non-graphics-bus-request) state it broadcasts a graphics-bus-request,BRG, (non-graphics-bus-request, NBRG) message to the other MLCTs. Thus,the NGP task knows the graphics bus state of all MLCTs in the call.

At the start of the conference, all MLCTs are in the NG-idle state. Ifone of the MLCTs active serving rooms requests graphics (transmits a STGmessage), the NGP task checks all of the other MLCTs graphics busstates. If at least one of the other MLCTs is in thegraphics-bus-request state the NGP task transmits astart-graphics-negative-acknowledge (STGNAK) message. That is to say, ifmore than one room in the conference has requested graphics videotransmission, all requests are denied. Otherwise, the NGP task puts theMLCT in the graphics-bus-request state, broadcasts a BRG message to theother MLCTs, and starts a timing interval window. When the timing windowexpires the NGP task checks to see if graphics bus contention exists.(i.e., Another of the MLCT's serving rooms is requesting graphics or oneof the other MLCTs has assumed the graphics-bus-request state). Ifgraphics bus contention exists the NGP task transmits a STGNAK messageto the serving room(s) requesting graphics, puts the MLCT in thenon-graphics-bus request state, and broadcasts a NBRG message to theother MLCTs. If graphics bus contention does not exist the NGP taskassigns the new graphics to the serving room (room I) requestinggraphics, puts the MLCT in the NG-this-MLCT state, broadcasts anew-graphics-is-room-I (NGI) message to the other MLCTs, and transmits astart-graphics-acknowledge (STGACK) to room I. Room I is now in thegraphics-only or graphics-and-talker state. The MLCT remains in theNG-this-MLCT state until one of the following events occurs:

1. The room assigned the new-graphics assumes the idle state ortalker-only state. In this case, the NGP task transmits a BLG message tothe other MLCTs and puts the MLCT in the NG-idle state.

2. The MLCT receives a BLG message. (This is an invalid condition, sinceonly the MLCT that serves the room assigned the new-graphics shouldtransmit BLG.) In this event, the NPG task would transmit an STGNAKmessage to the room that was assigned the new-graphics and put the MLCTin the NG-idle state.

3. The MLCT receives an NGJ message. (This is an invalid condition,since no MLCT should assign a new-graphics when the new-graphics isalready assigned.) In this event, the MLCT would broadcast a BLG messageto all the other MLCTs to force them into the NG-idle state, transmit anSTGNAK message to the room that was assigned the new-graphics, and enterthe NG-idle state.

If the MLCT is in the NG-idle state and receives an NGI message fromanother MLCT, the NGP task puts the MLCT in the NG-other-MLCT state. Ifthe MLCT is in the NG-other-MLCT state and should receive an NGJ message(this is an invalid condition), the NGP task would assume thenew-graphics is assigned to room J. When the MLCT in the NG-other-MLCTstate receives a BLG message, the NGP task puts the MLCT in the NG-idlestate.

If the MLCT is in the stand-alone condition then the NGP task does notcheck the other MLCTs graphics-bus states. It instead performs theassignment of the new graphics as if this MLCT is the only MLCT in thecall.

The 400 millisecond timing window used to resolve graphics contentionshould prevent more than one MLCT from assigning the new graphics at atime. If an MLCT has assigned the new graphics (is in the NG-this-MLCTstate) and receives a NGI message from another MLCT, the first MLCTtransmits a BLG message placing all MLCTs back into the NG-idle state.

FIGS. 53, 54, 55 and 56 connected as shown depict a flowchart of thesteps performed in the new graphics process (NGP) task by controlprocessor 1006 of FIG. 39. This program is also stored in memory 3904(FIG. 39).

The NGP task maintains the new graphic state and graphic bus state ofthe MLCT. It processes changes to the rooms served by the MLCT, i.e.,active and graphics and the new graphics and graphics bus states of theother MLCTs in the conference and updates its new graphics stateaccordingly.

The NGP task is entered via oval 5301. Thereafter, operational block5302 causes the NGP task to initialize the variables it controls,namely, new graphics=unassigned, graphics bus=non graphics bus request,graphid=unassigned, transmits a bus graphics (BLG) message, transmit abus request graphics (BRG) message, and wait-for-post.

Operational block 5303 updates the graphic bus state (GBUS) of the MLCTand transmits a BRG/NBRG message if a change in the graphic bus stateoccurs.

Conditional branch point 5304 tests to determine if the new graphic (NG)is assigned. If the test is true, control is transferred to conditionalbranch point 5305. If the test is false, control is transferred toconditional branch point 5306. Conditional branch point 5306 tests todetermine if a NGI message has been received and if it should beprocessed. If the test is false, control is transferred to conditionalbranch point 5307. If the test is true, control is transferred toconditional branch point 5308.

Conditional branch point 5308 tests to determine if the NGI messageassigns the new graphics to a room served by this MLCT. If the testresult in step 5308 is true, control is transferred to operational block5309 which transmits a bus release message (BL). Thereafter, control istransferred to conditional branch point 5307. If the test result in step5308 is false, the room is not served by this MLCT and control istransferred to operational block 5346.

Operational block 5346 assigns the new graphics to the room specifiedand posts the VSEL task which is shown in FIGS. 58 and 59 and describedbelow.

Operational block 5310 sets the graphid to unassigned. Conditionalbranch point 5311 tests to determine if this MLCT's graphics bus=BRG. Ifthe test is true, operational block 5312 calls sub-routine GRACK shownin FIG. 57 and described below. Then, control is transferred tooperational block 5313. If the test result in step 5311 is false,control is transferred directly to operational block 5313 which causes await-for-post. Thereafter, control is returned to operational block5303.

Returning to conditional branch point 5307, it tests to determine if theMLCT has one-and only-one serving room active and requesting graphics.If the test is false, control is transferred to operational block 5314.However, if the test is true, control is transferred to conditionalbranch point 5315. Conditional branch point 5315 tests to determine ifthe graphics bus state of all the other MLCTs=non graphics bus request.If the test is false, control is transferred to operational block 5314.However, if the test is true, control is transferred to conditionalbranch point 5316. Conditional branch point 5316 tests to determine ifthe graphid=the id of the room requesting graphics. If the test isfalse, control is transferred to operational block 5317. Operationalblock 5317 sets the new graphics=to the id of the room requestinggraphics. Then, operational block 5318 causes a wait interval of 400milliseconds. Thereafter, control is returned to operational block 5303.

If the test result in conditional branch point 5316 is true, theoperational block 5319 sets the new graphics=to the room id stored ingraphid, transmits a NGI message to the other MLCTs, and posts the VSELtask noted above.

Operational block 5314 sets graphid=to unassigned. Then, conditionalbranch point 5320 tests to determine if this MLCT's graphic bus=graphicsbus request. If the test is true, control is transferred to operationalblock 5321 which calls the GRACK sub-routine, noted above. Then, controlis transferred to operational block 5322. If the test in step 5320 isfalse, control is transferred directly to operational block 5322.Operational block 5322 causes a wait-for-post to occur. Then, control isreturned to operational block 5303.

Conditional branch point 5305 tests to determine if the new graphics isassigned to a room served by this MLCT. If the test is false, control istransferred to conditional branch point 5323. However, if the test istrue, control is transferred to conditional branch point 5324.Conditional branch point 5324 tests to determine if the MLCT is in thestand-alone condition. If the test is true, control is transfereed toconditional branch point 5325. However, if the test result in step 5324is false, conditional branch point 5326 tests to determine if a BLGmessage has been received. If the test is true, control is transferredto operational block 5327. However, if the test result in step 5326 isfalse, control is transferred to conditional branch point 5328.Conditional branch point 5328 tests to determine if a NGI message hasbeen received from another MLCT. If the test is false, control istransferred to conditional branch point 5325. However, if the testresult in step 5328 is true, control is transferred to operational block5329.

Conditional branch point 5325 tests to determine if the new graphicsroom is active and still requesting graphics. If the test is true,control is transferred to operational block 5330. If the test is false,control is transferred to operational block 5329 and a BLG message istransmitted to the other MLCTs. Operational block 5327 posts the VSELtask, calls the GRACK sub-routine and sets the graphid=unassigned.Operational block 5331 causes a wait-for-post. Thereafter, control istransferred to operational block 5303.

Operational block 5330 sets graphid=unassigned. Conditional branch point5332 tests to determine if only one room served by the MLCT is activeand requesting graphics. If the test is false, control is transferred tooperational block 5333 which causes the GRACK sub-routine. Control isthen transferred to operational block 5334. If the test in step 5332 istrue, control is transferred directly to operational block 5334 whichcalls a wait-for-post. Thereafter, control is returned to operationalblock 5303.

Conditional branch point 5323 tests to determine if the MLCT is in thestand-alone-condition. If the test is true, control is transferred tooperational block 5335. Operational block 5335 causes a BLG message tobe transmitted to the other MLCTs, sets new graphics=to unassigned, andposts the VSEL task. Control is then transferred to operational block5336. If the test result in step 5323 is false, control is transferredto conditional branch point 5337. Conditional branch point 5337 tests todetermine if a BLG message has been received from another MLCT. If thetest is true, control is transferred to operational block 5338. If thetest result in step 5337 is false, the control is transferred toconditional branch point 5339 which tests to determine if a new NGImessage has been received from another MLCT. If the test in step 5339 isfalse, control is transferred to operational block 5336. If the testresult in step 5339 is true, control is transferred to conditionalbranch point 5340. Conditional branch point 5340 tests to determine ifthe NGI message assigns the new graphics to a room served by this MLCT.If the test is false, control is transferred to operational block 5341which assigns the new graphics=to the room specified by the NGI messageand posts the VSEL task. Control is then transferred to operationalblock 5336. If the test result in step 5341 is true, operational block5342 transmits a BLG message to the other MLCTs. Then, operational block5338 sets the new graphics=unassigned and posts the VSEL task.Operational block 5336 sets graphid=unassigned.

Conditional branch point 5343 tests to determine if this MLCT's graphicsbus=bus request graphics. If the test is false, control is transferredto operational block 5344 which calls the GRACK sub-routine noted above.Then control is transferred to operational block 5345. If the test in5343 is true, control is transferred directly to operational block 5345which waits for post. Thereafter, control is returned to operationalblock 5303.

FIG. 57 shows a flowchart of the steps performed in the GRACKsub-routine utilized in the new graphics processing task of FIGS. 53,54, 55 and 56 and performed by control processor 1006 (FIG. 39). Theprogram described by this flowchart is stored in memory 3409 (FIG. 39).

The GRACK sub-routine processes the active serving rooms requestinggraphics after new graphics have been assigned. It sends the appropriateSTGACK/STGNAK message to each of the rooms clears the graphics requestsfor those rooms, which were denied graphics and updates the graphics busin this MLCT.

The GRACK sub-routine is entered via oval 5701. Thereafter, operationalblock 5702 sets TEMPGBUS=non graphics bus request and sets counter J=5.Conditional branch point 5703 tests to determine if room J is requestinggraphics. If the test is false, then control is transferred tooperational block 5704. If the test is true, control is transferred toconditional branch point 5705. Conditional branch point 5705 tests todetermine if room J is the new graphics room. If the test is true,operational block 5706 sets TEMPGBUS=graphics bus request and transmitsa STGACK message to room associated with the id J. Then, control istransferred to operational block 5704. If the test result in step 5705is false, operational block 5707 clears the request for graphics for theroom identified by J and transmits a STGNAK message to that room.Control is then transferred to operational block 5704 which sets counterJ to J-1. Conditional branch point 5708 tests to determine if J is lessthan 0. If the test is false, control is transferred to conditionalbranch point 5703 and the process is iterated until the test result instep 5708 is true. Then, operational block 5709 sets this MLCT graphicsbus=TEMPGBUS bus. Control is then returned to the new graphics task viaoval 5710.

Video Selection (VSEL) Task

Video Selection (VSEL) task 4703 selects the video signal transmitted toeach room in the conference according to the mode that the room is in.In a viewer's choice conference a room may be in either the push-to-see(PTS) mode or the automatic video switching (AVX) mode. The roomcontrols the mode it is in by the messages that it transmits to theMLCT. In a Sender's Choice conference all rooms remain in the AVX mode.

When an MLCT in a Viewer's Choice conference receives apush-to-see-room-I (PTSI) message from an active room (room J) to selectanother active room (room I), the VSEL task places room J in the PTSmode and performs the steps required to switch the video of room J toview room I. The get-ready-for-fast-update-from-room-I (GRI) messagethat room J receives from room I functions as the acknowledge to roomJ's PTSI message.

If an MLCT in a Viewer's Choice conference should receive a PTSI messageselecting a room that is inactive, the message processing module wouldtransmit an available-rooms (AR) message to the room. This ensures thatthe room would have the current available rooms list and also serves asthe negative acknowledge to the PTSI message. The room that hastransmitted the invalid PTSI message would remain in its previous PTS orAVX viewing mode.

In a Sender's Choice conference the message processing module respondsto all PTS messages by transmitting an AR message with an empty list ofavailable rooms. Thus, from the video selection viewpoint the Sender'sChoice conference can be treated as a special Viewer's Choiceconference.

A room can switch from the PTS mode to the AVX mode by transmitting anautomatic-video-switch (AVX) message. The VSEL task generates an AVXacknowledge (AVXACK) message to the room and puts the room in the AVXmode. The VSEL task also puts any inactive room that it serves in theAVX mode.

The VSEL task selects the input video for every room in the callaccording to the following priorities:

1. To view the selected room--If the room is in the PTS mode or entersthe PTS mode, the room receives the video from the room it specified inthe last PTSi message.

2. To view the new-graphics--If the room is in the AVX mode and ifanother room is assigned the new graphics, the room receives the videofrom that room.

3. To view the new-talker--If the room is in the AVX mode, and ifanother room is assigned the new talker, the room receives the videofrom that room.

4. To view the last new-talker--If the room is in the AVX mode, and ifanother room was the last new talker, the room receives the video fromthat room.

5. To view the second to last new-talker--If the room is in the AVXmode, and if another room was the second to last new talker, the roomreceives the video from that room.

6. If none of the above conditions apply, no video switch is made. Theroom continues to receive the video it is receiving.

The VSEL task is posted by the following:

Facility Administrator Module: Posts VSEL task when it changes theactive status of one or more rooms.

Message Processing Module: Posts VSEL task when it adjusts one or moreof the room's PTS modes.

NTP Task: Posts VSEL task when it changes the assignment of the newtalker.

NGP Task: Posts VSEL task when it changes the assignment of the newgraphics.

FIGS. 58 and 59 connected as shown depict a flowchart of the stepsperformed in the video selection (VSEL) task by control processor 1006of FIG. 39. The program is stored in memory 3904 (FIG. 39).

The VSEL task determines the desired video of each room in a conferenceand the desired video of the undedicated MLCT broadcast links.

Accordingly, the VSEL task is entered via oval 5801. Thereafter,operational block 5802 initializes the variables controlled by the VSELtask, namely, it clears all fast update expected flags of the rooms inthe conference and clears all fast update for video select flags for allrooms in the conference. Operational block 5803 causes a wait-for-postand operational block 4 sets counter J=5. Five is employed in thisexample because there are a maximum of 6 rooms in a conference.

Conditional branch point 5805 tests to determine if room J is active. Ifthe test is false, operational block 5806 puts room J in the automaticvideo switching mode (AVX). Thereafter, control is transferred toconditional branch point 5807. If the test in step 5805 is true, controltransfers directly to conditional branch point 5807.

Conditional branch point 5807 tests to determine if room J is assigned.If the test is true, control is transferred to conditional branch point5808. However, if the test is false, control is transfereed tooperational block 5809. Conditional branch point 5808 tests to determineif room J is in the push-to-see (PTS) mode and is selecting an activeroom. If the test is true, operational block 5810 sets room J's desiredvideo equal to the selected PTS room. Thereafter, control is transferredto conditional branch point 5812. If the test result in step 5808 isfalse, operational block 5811 places room J in the AVX mode.

Conditional branch point 5813 tests to determine if the new graphics(NG) is assigned to another room. If the test is true, operational block5814 sets room J's desired video equal to the new graphics room.Thereafter, control is transferred to conditional branch point 5811. Ifthe test in step 5813 is false, conditional branch point 5815 tests todetermine if the new talker (NT) is assigned to another room. If thetest is true, operational block 5816 sets room J's desired video equalto the new talker room. Thereafter, control is transferred toconditional branch point 5811.

If the test in step 5815 is false, conditional branch 5817 tests todetermine if the last new talker is assigned to another room. If thetest is true, operational block 5818 sets room J's desired video equalto the last new talker. Thereafter, control is transferred toconditional branch point 5811. If the test in step 5817 is false,conditional branch 5819 tests to determine if the second to last newtalker is assigned to another room in the conference. If the test istrue, operational block 5820 sets room J's desired video equal to thesecond to last new talker. Thereafter, control is transferred toconditional branch point 5811. If the test in step 5819 is false,control is transferred directly to conditional branch point 5811.

Conditional branch point 5811 tests to determine if room J's desiredvideo equals its present video. If the test is true, control istransferred to operational block 5809. However, if the test is false,conditional branch point 5821 tests to determine if room J's desiredvideo equals a room served by this MLCT. If the test is true,operational block 5822 sets the desired room fast update for videoselect flag and transfers control to operational block 5823. If the stepin 5821 is false , operational block 5824 sets the desired rooms fastupdate expected flag and control is transferred to operational block5823. Operational block 5823 causes a post to the FUGEN task. A flowchart of the FUGEN task is shown in FIGS. 60 and 61, and is describedbelow. Thereafter control is transferred to operational block 5809.Operational block 5809 sets counter J=J-1.

Conditional branch point 5825 tests to determine if counter J is lessthan 0. If the test is false, control is transferred to conditionalbranch point 5805. If the test is true, conditional branch 5826 tests todetermine if the new graphics is assigned to a room served by this MLCT.If the test is true, operational block 5827 sets the desired video ofthe undedicated MLCT broadcast links equal to the new graphics.Thereafter, control is transferred to operational block 5803.

If the test in step 5826 is false, conditional branch 5828 tests todetermine if the talker is assigned to a room served by this new MLCT.If the test in step 5828 is false, control is transferred directly tooperational block 5803. However, if the test is true, operational block5829 sets the desired video of the undedicated MLCT broadcast linksequal to the new talker. Thereafter control is transferred tooperational block 5803.

Video Switching and Fast Update Algorithm

The video switching and fast update algorithm is implemented in the FastUpdate Generator (FUGEN) task 4704, Fast Update Monitor (FUMON) task4705, and Video Switch (VSWCH) task 4706. The FUGEN task 4704 generatesfast video picture updates according to which of the room's fast updateflags are set and what state the room's fast update timers are in. Whenthe FUGEN task 4704 starts any of the room's timers it posts the FUMONtask 4705.

The fast updates are required when switching because the digital signalusually only includes changes in the picture scenes in order to compressthe transmission bandwidth.

The FUMON task 4705 operates the room's fast update timers. These fastupdate timers are actually counters that the FUMON task 4705 decrementsevery 250 milliseconds. When any of the room's fast update timersexpires (i.e. the counter is decremented to zero) and fast updates needto be generated, then the FUMON task 4705 posts the FUGEN task 4704.

The VSWCH task 4706 processes fast updates when they are received. Itnotifys the FUMON task 4705 that a fast update was processed and sendsan interrupt to the MS 101 if a video switch is required.

The MLCT must synchronize the switching of the video signal beingtransmitted to a room with a fast update of the new video signal. Thefast update provides the room with information of the complete videopicture. Without this fast update the room's video would be severelyaffected for a number of seconds after the switch has occurred. The fastupdate must follow the video switch with enough delay to allow theroom's TV picture processor to synchronize to the framing of the newvideo signal. This delay should be minimized to reduce the time that theroom's video picture is frozen or blanked when the video switch occurs.

The MLCT can reduce the number of fast updates required during theconference if it monitors fast updates that it processes. Fast updaterequests can be dropped if the MLCT determines that they can be handledby fast updates pending or already processed. This is important becausethe fast update initially provides a low resolution video picture. Thisis achieved, in accordance with an aspect of the invention, by storingthe video select modes, i.e., Push-to-See (PTS) or Automatic VideoSwitch (AVX), and the transmission states for each conference locationand utilizing the stored information to determine any differencesbetween the video signals presently being transmitted to the locationsand those to be transmitted. Fast video picture updates are generatedonly for those locations for which there is a difference between thepresently transmitted and the to be transmitted video signals.

To perform the video switch (room J to view room I), the MLCT transmitsa generate-fast-update-command (GFUC) message to room I if the MLCT alsoserves room I. If room I is not served by this MLCT, the MLCT doesnothing. The MLCT that serves room I will transmit the GFUR commandmessage to generate the fast update. Room I will respond by transmittinga get-ready-for-fast-update (GR) message, wait a specified period oftime, and then send a fast update of its video signal.

Upon receipt of the GR message from room I the MLCT serving room I will:

1. Perform any video switches required for rooms viewing room I.

2. Transmit a GRI message to its serving rooms that are receiving roomI's video signal.

3. Transmit a GRI message over the broadcast links that are transmittingroom I's video signal.

Upon receipt of the GRI message from another MLCT this MLCT will:

1. Perform any video switches required for rooms viewing room I.

2. Transmit a GRI message to its serving rooms that are receiving roomI's video signal.

The acknowledgement to the GFUC (GFURIJ) message is the GR (GRI) messagereceived from room I (the other MLCT).

The delay between the room's transmission of the GRI message and theactual start of the fast update should be long enough to allow thefollowing events to occur:

1. The first MLCT receives and interprets the GR command received fromthe room (room I).

2. The first MLCT performs a video switch.

3. The first MLCT transmits a GRI message to other MLCTs that areexpecting to receive the video.

4. The other MLCT(s) receive and interpret the GRI command received fromthe first MLCT.

5. The second MLCT(s) perform a video switch.

6. The other MLCT(s) transmit a GRI message to rooms receiving room I'svideo. This transmission does not have to precede the actual fastupdate.

7. The rooms that have just been switched to receive room I's video mustsynchronize to the encryption and framing of the new video signal. Thetime required to synchronize varies according to the transmission rate,for example 140 milliseconds for a 6.0 Mhz signal and 240 millisecondsfor a 3.0 Mhz signal

The time requird to allow the MLCTs to process the GR/GRI messages,perform video switches, and transmit GRI messages is estimated to beunder 250 milliseconds for most cases.

The MLCT maintains a wait-for-fast-update timer anddisregard-fast-update-requests timer for each room in the call. Thesetimers are actually counters that are set to a positive value anddecremented once every 250 milliseconds until they reach zero. Whilepositive the timer counter is considered to be active.

When the MLCT transmits a GFUC or GFURI message requesting a fast updatefrom room I it starts both of room I's fast update timers. While theroom I's disregard-fast-update-requests timer is active the MLCT willdrop, i.e., disregard, all requests from rooms for a fast update fromroom I. This enables the MLCT to service multiple requests with one fastupdate. Examples of when multiple requests are serviced by one fastupdate are:

1. The MLCT switches the video going to several rooms to view the newtalker. A fast update from the new talker is pending, but the roomsdetect loss of video synchronization and transmit agenerate-fast-update-request (GFUR) message to the MLCT before receivingthe fast update.

2. Several rooms are viewing the new talker room when that room'soutgoing video is disrupted. Each of the rooms viewing the new talkerroom detects loss of synchronization and transmits agenerate-fast-update-request (GFUR) message to the MLCT, but only onefast update is required.

The room's wait-for-fast-update timer should be greater than or equal tothe room's disregard-fast-update-requests timer. When the room'sdisregard-fast-update-requests timer expires the MLCT holds all requestsfor fast updates, i.e., delays responding to the requests, until thewait-for-fast-update timer expires. In one example, the wait-for-fastupdate interval is equal to the sum of the disregard and hold intervals.

It is noted, that the room's disregard fast update timer is reinstatedwhen a GRGRI message is received from or for that room. The MLCT alsomaintains three request fast update flags for each room in the call.This allows separation of the requests for fast updates according totheir source. The room's request for fast update flags are:

1. Room I's request-fast-update-received flag is set when the MLCTreceives a GFUR message from a room viewing room I or a GFURI messagefrom another MLCT.

2. Room I's request-fast-update-for-video-switch flag is set when theMLCT determines that a fast update from room I is required in order toperform a video switch.

3. Room I's fast-update-expected flag is set when room I is served byanother MLCT and one of this MLCT's serving rooms and should be viewingroom I. The other MLCT serving room I should generate the fast updateautomatically. Setting this flag will cause this MLCT to start room I'sfast update timers in anticipation of a fast update from that roomwithout actually requesting the fast update.

If the fast-update-expected flag is set for a room served by anotherMLCT, this MLCT starts that room's two fast update timers, but does nottransmit a GFURI message. The MLCT processes the room'srequest-fast-update-received and request-fast-update-for-video-switchflags as follows:

1. If room I's disregard-fast-update-requests timer is active, then theMLCT clears the request-fast-update-received flag of any room viewingroom I. The MLCT does nothing with therequest-fast-update-for-video-switch flag of any room viewing room I.These flags are processed later.

2. If room I's disregard-fast-update-requests timer is inactive andwait-for-fast-update timer is active, the MLCT does nothing with therequest-fast-update-received or request-fast-update-for-video-selectflags of rooms viewing room I. These flags are processed later.

3. If room I has both fast update timers inactive, the MLCT checks tosee if any room viewing room i has their request-fast-update-received orrequest-fast-update-for-video-select flag set. If a flag is set the MLCTclears the flag and transmits a GFUC/GFURI message.

The Fast Update Generator (FUGEN) task 4704 generates video fast updatesfrom rooms. It generates the required GFUC/GFURI messages to betransmitted to rooms/MLCTs, starts the rooms fast update timers, andposts the FUMON task 4705 to monitor the timers.

The Fast Update Monitor (FUMON) task 4705 operates the room's fastupdate timer counters. The Video Switch (VSWCH) 4706 task processes thefast updates as they arrive. To this end, broadcast links are dedicatedto ensure that the MCLT will be able to transmit any of the requiredcombination of video signals to the other MCLTs. At the time ofconference set up the facility administrator module decides how todedicate each of its broadcast links as described above. The dedicationof these broadcast links will not be changed unless the call set up ischanged.

In a Sender's Choice call each MLCT requires only one broadcast link.This broadcast link is not dedicated.

In a Viewer's Choice call MLCTs may need more than one broadcast link.The push-to-see (PTS) mode requires that MLCTs be able to transmit anycombination of video signals to the rooms. In a Viewer's Choice call thebroadcast links are dedicated by the facility administrator moduleaccording to following priorities:

1. If the MLCT has enough broadcast links it assigns a broadcast link toeach of its serving rooms. In this case all of the other MLCTs alwaysreceive video from every room served by this MLCT.

2. If the MLCT has enough broadcast links it assigns a broadcast link toeach of the rooms served by the other MLCTs. In this case the MLCT willalways be able to transmit the video required for each of the roomsserved by other MCLTs.

3. If the MLCT does not have enough broadcast links to satisfy either oftwo previous cases, then the MLCT cannot operate properly in the call.

If the MLCT has more than enough broadcast links needed for dedicationto the rooms, then it leaves the extra broadcast links undedicated.

The VSWCH task 4706 prcesses the fast updates when they arrive.

Fast Update Generator (FUGEN) Task

FIGS. 60 and 61 connected as shown depict a flowchart of the stepsperformed in the fast update generator (FUGEN) task by controllerprocessor 1006 of FIG. 39. The program is stored in memory 3904 (FIG.39).

The fast update generator (FUGEN) task initiates the fast update to begenerated by the conference room video processor. It generates requestfast update (FU) messages to be transmitted to rooms and other MLCTs inthe conference. It checks the fast update related flags and timingcounters of the rooms to determine if a fast update should be requested.The FUGEN task also starts the room's fast update counters if fastupdates are expected.

Accordingly, the FUGEN task is entered via oval 6001. Then, operationalblock 6002 causes a wait-for-post. post. Operational block 6003 setscounter J=5. Conditional branch point 6004 tests to determine if room Jis active. If the test is false, operational block 6005 clears room J'srequest fast update received flag, fast update for video select flag andfast updated expected flag. Thereafter, control is transferred tooperational block 6006.

If the test in step 6004 is true, i.e., room J is active, conditionalbranch point 6007 tests to determine if room J's request fast updatereceived flag or fast update for video select flag is set. If the testin step 6007 is false, control is passed to conditional branch point6008. However, if the test is true, conditional branch point 6009 teststo determine if room J's wait-for-fast-update counter is 0. If the testis false, conditional branch point 6010 tests to determine if room J'sdisregard-fast-update-requests counter=0. If the test in step 6010 isfalse, operational block 6011 clears room J's fast update received flag.Thereafter, control is transferred to conditional branch point 6008. Ifthe test in step 6010 is true, control is transferred directly toconditional branch point 6008.

If the test result in step 6009 is true, operational block 6012 clearsroom J's request fast update received flag, fast update for video selectflag and fast update expected flag.

Conditional branch point 6013 tests to determine if room J is served bythis MLCT. If the test is true, operational block 6014 causes the GFUCmessage to be transmitted to room J, sets room J'sdisregard-fast-update-requests counter=4, and room J'swait-for-fast-update counter=4. Thereafter, control is transferred tooperational block 6115. If the test result in 6013 is false, operationalblock 6016 causes a GFURJ message to be transmitted to the MLCT servingroom J, sets room J's disregard-fast-update requests counter=20 and setsroom J's wait-for-fast-update counter=20. Then, operational block 6015causes a post to the fast update monitor (FUMON) task. A flowchart ofthe FUMON task is shown in FIGS. 62 and 63 and described below.Thereafter, control is transferred to conditional branch point 6008.

Conditional branch point 6008 tests to determine if room J's fast updateexpected flag is set. If the test is false, control is transferred tooperational block 6006. If the test is true, conditional branch point6017 tests to determine if room J is served by this MLCT. If the test istrue, operational block 6018 clears room J's fast update expected flagand transfers control to operational block 6006. If the test in step6017 is false, operational block 6019 clears room J's fast updateexpected flag, fast update for video select flag, and request fastupdate received flag, and then sets room J's disregard-fast-updaterequests counter=20, wait-for-fast-update counter=20, and posts theFUMON task. Thereafter, control is transferred to operational block6006.

Operational block 6006 sets counter J=J-1. Conditional branch point 6020tests to determine if counter J is less than 0. If the test is false,control is transferred to conditional branch point 6004. If the test istrue, control is transferred to operational block 6002.

Fast Update Monitor (FUMON) Task

FIGS. 62 and 63 connected as shown depict a flowchart of the stepsperformed in the fast update monitor (FUMON) task by control processor1006 of FIG. 39. The program is stored in memory 3904 FIG. 39.

When posted by other tasks, the (FUMON) task enters a loop ofpredetermined interval, for example, 250 milliseconds, decrementing thefast update (FU) timing counters of the rooms. When a roomswait-for-fast-update timer expires, i.e., reaches a count of 0 and afast update is required for a corresponding room video processor, theFUMON task posts the FUGEN task to initiate the fast update. When thefast-update counters of all the rooms in the conference have expired,the FUMON task retires and waits for another post.

Accordingly, the FUMON task is entered via oval 6201. Thereafteroperational block 6202 causes a wait-for-post. Operational block 6203clears the FUMON active flag and sets counter J=6. Conditional branchpoint 6204 tests to determine if room J's fast update process flag isset. If the test is true, operational block 6205 clears room J's fastupdate process flag, sets room J's wait-for-fast-update count=16, andsets FUMON task active flag. Thereafter, control is transferred tooperational block 6206.

If the test in step 6204 is false, conditional branch point 6207 teststo determine if room J's wait-for-fast-update counter is 0. If the testis true, control transfers directly to operational block 6206. If thetest is false, operational block 6208 sets the FUMON active flag.Operational block 6209 decrements room J's wait-for-fast-update counterby one. Conditional branch point 6210 tests to determine if room J'swait-for-fast-update counter is 0. If the test is false, controltransfers directly to conditional branch point 6211. If the test istrue, conditional branch point 6212 tests to determine if room J's fastupdate expected flag, request fast update received flag or fast updatefor video select flag is set. If the test in step 6212 is true,operational block 6213 causes a post to the FUGEN task. A flowchart ofthe FUGEN task is shown in FIGS. 60 and 61 and is described above.Thereafter, control is transferred to conditional branch point 6211.

If the test in step 6212 is false, control transfers directly toconditional branch point 6211. Conditional branch point 6211 tests todetermine if room J's disregard-fast-upate requests counter is 0. If thetest is false, operational block 6214 decrements room J'sdisregard-fast-update requests counter and transfers control tooperational block 6206. If the test in step 6211 is true, controltransfers directly to operational block 6206 which sets counter J=J-1.

Conditional branch 6215 tests to determine if J is less than 0. If thetest is false, control is transferred to conditional branch point 6204.If the test is true, conditional branch point 6216 tests to determine ifthe FUMON active flag is set. If the test is true, operational block6217 causes a wait for 250 milliseconds. Thereafter, control istransferred to operational block 6203. If the test in step 6216 isfalse, operational block 6218 causes a wait-for-post and transferscontrol to operational block 6203.

Video Switch (VSWCH) Task

FIGS. 64 and 65 connected as shown depict a flowchart of the stepsperformed in the video switch task (VSWCH) by control processor 1006 ofFIG. 39. The program is stored in memory 3904 of (FIG. 39).

The VSWCH task processes the in-links fast update received flags. Themessage processing module of control processor 1006 sets this in-linksflag and posts the VSWCH task when it receives aget-ready-for-fast-update from room I message (GRI) over that particularin-link. The VSWCH task determines which outgoing links should betransmitting the particular in-links video signal and notifies themicroprocessor system 101 if a video switch is required. The VSWCH taskalso transmits a GRI message over each of those outgoing linkstransmitting the particular in-links video, sets the fast updateprocessed flag for room I, initiates the fast update disregard counterfor room I and posts the FUMON task described in relationship to FIGS.62 and 63.

Accordingly, the VSWCH task is entered via oval 6401 and, then,operational block 6402 causes a wait for post and sets counter K=5.Conditional branch point 6403 tests to determine if in-link K isassigned. If the test is false, control is transferred to operationalblock 6404. If the test is true, conditional branch point 6405 tests todetermine if in-link K's fast update received flag is set. If the testis false, control is transferred directly to operational block 6404. Ifthe test in step 6405 is true, operational block 6406 sets counter J=5.

Conditional branch point 6407 tests to determine if outgoing link J isassigned. If the test is false, control is transferred to operationalblock 6408. If the test is true, conditional branch point 6409 tests todetermine if in-link K is a room link. If the test is true control istransferred to conditional branch point 6410. If the test is false,conditional branch point 6411 tests to determine if out-link J is a roomlink. If the test is false, control is transferred to operational block6408. If the test is true, conditional branch 6412 tests to determine ifroom B's desired video=room A. Room A is defined as the room originatingthe present video transmitted on in-link K. Room B is defined as theroom that out-link J is dedicated to.

If the test in step 6412 is false, control is transferred to operationalblock 6408. If the test in step 6412 is true, conditional branch point6413 tests to determine if the MLCT is operating in a Sender's choiceconference mode. If the test is true, control is transferred tooperational block 6414. If the test is false, conditional branch point6415 tests to determine if in-link K is dedicated to room A or B. If thetest is true, operational block 6414 sets room B's present video=to roomA and calls the SWITCHLK sub-routine. The flowchart of the SWITCHLKsub-routine is described in FIG. 66 and described below. Thereafter,control is transferred to operational block 6408. If the test in step6415 is false, control is transferred to operational block 6408.

Conditional branch point 6410 tests to determine if out-link J is a roomlink. If the test is true, control is transferred to conditional branchpoint 6416. If the test is false, conditional branch point 6417 tests todetermine if out-link J is dedicated. If the test is true, control istransferred to conditional branch point 6418. If the test is false,conditional branch point 6419 tests to determine if the desired video ofthe undedicated MLCT broadcast links=room A. If the test is false,control is transferred to operational block 6408. If the test is true,control is transferred to operational block 6420. Operational block 6420sets the present video=to room A for all rooms served by other MLCTs whohave their desired video=to room A and calls the SWITCHLK sub-routine.Control is then transferred to operational block 6408.

Conditional branch point 6418 tests to determine if out-link J isdedicated to room A. If the test result is true, control is transferredto operational block 6420 as described above and then to operationalblock 6408. However, if the test result is false, control is transferredto conditional branch point 6421 which tests to determine if out-link Jis dedicated to a room served by this MLCT. If the test is true, controlis transferred to operational block 6408. If the test is false,conditional branch point 6422 tests to determine if the desired video ofthe room that out-link J is dedicated to is room A. If the test isfalse, control is transferred to operational block 6408. If the test istrue, operational block 6423 sets the present video of the room thatout-link J is dedicated to to room A and calls the SWITCHLK sub-routine.Thereafter, control is transferred to operational block 6408.

Operational block 6408 sets room A's fast update processed flag and roomA's disregard-fast-update-requests counter=4 and sets counter J=J-1.Conditional branch point 6425 tests to determine if counter J is lessthan 0. If the test is false, control is transferred to conditionalbranch 6407. If the test is true, control is transferred to operationalblock 6404.

Operational block 6404 sets counter K=K-1. Conditional branch point 6426tests to determine if K is less than 0. If the test is false, control istransferred to conditional branch point 6403. If the test is true,control is transferred to operational block 6402.

FIG. 66 shows a flowchart of the steps performed in the (SWITCHLK)sub-routine used in the VSWCH task program and performed by controlprocessor 1006 of FIG. 39. The program is stored in memory 3904 (FIG.39).

Sub routine SWITCHLK selects in-link K's video for transmission onout-link J.

Accordingly, the sub-routine is entered via oval 6601. Thereafter,operational block 6602 causes a GRI message to be transmitted overout-link J and sets out-link J's present video=to room A. Conditionalbranch point 6603 tests to determine if out-link J selectedin-link=in-link K. If the test is false, operational block 6604 setsout-link J's selected in-link to in-link K and sends the microprocessorsystem a message to perform a video switch so that out-link K transmitsin-link K's video signal. Control is then transferred back to the VSWCHtask via oval 6605. If the test in step 6603 is true, control istransferred directly back to the VSWCH task via oval 6605.

FIGS. 67 and 68 when connected as shown form a flow chart of stepsperformed in the facility administrator task (FADMIN) 4708 (FIG. 47) bycontrol processor 1006 of FIG. 39. The program is stored in memory 3904(FIG. 39).

Briefly, the FADMIN task maintains the active and muted states of theconference rooms assigned to the MLCT, maintains an available rooms list(AR) and performs the dedication of outgoing transmission links, i.e.,paths, to other MLCTs.

Accordingly, the FADMIN task is entered via oval 6701. Thereafter,control is transferred to operational block 6702 which executes a waitfor post, initializes variables, and sets counter J=5.

Operational block 6703 declares outgoing link J active if it is assignedand control channel communication is established. Otherwise, outgoinglink J is declared inactive. It also declares incoming link J active ifit is assigned and the control channel communication is established.Otherwise, incoming link J is declared inactive.

Conditional branch point 6704 tests to see if incoming link J andoutgoing link J are assigned to a room. If the test result is true,control is passed to operational block 6705 which sets that room's mutedstate to the same state as incoming link J's muted state. Operationalblock 6705 also marks that room as active if both incoming link J andoutgoing link J are active. Otherwise, it marks that room inactive.Thereafter, control is transferred to operational block 6706 If the testresult of conditional branch point 6704 is false, control is transferredto conditional branch point 6707.

Conditional branch point 6707 tests to determine if either incoming linkJ's or outgoing link J's active state was just changed by operationalblock 2703. If the test result is false, control is passed tooperational block 6706. If the test result is true, control is passed tooperational block 6708.

Operational block 6708 generates a MSTAT message to be sent to the otherMLCTs and posts the NTP, NGP, and VSEL tasks. Thereafter, control istransferred to operational block 6706.

Operational block 6706 decrements counter J by one.

Conditional branch point 6709 tests to determine if counter J is lessthan zero. If the test result is false, control is passed to operationalblock 6703 and steps 6703 through 6708 are repeated until conditionalbranch point 6709 yields a true result. If the test result ofconditional branch point 6709 is true, control is passed to operationalblock 6710 which forms the available rooms list. A room is determined tobe available if it is assigned and is active.

Conditional branch point 6711 tests to determine if the available roomslist has changed. If the test is true control is passed to operationalblock 6712. Operational block 6712 generates an MSTAT message to be sentto the other MLCTs, generates an AR message to send to the rooms, andposts the NTP, NGP, and VSEL tasks. Control is then passed toconditional branch point 6713. If the test result of conditional branchpoint 6711 is false, control is transferred directly to conditionalbranch point 6713.

Conditional branch 6713 tests to determine if the MLCT is in the standalone condition. If the test result is true, operational block 6714undedicates all broadcast links. Thereafter, control is returned tooperational block 6702. If the test result of conditional branch point6713 is false, control is transferred to conditional branch point 6716.

Conditional branch point 6716 tests to determine if the number ofbroadcast links is sufficient for dedicating the links to the roomsassigned to this MLCT. If the test result is true, operational block6717 dedicates a broadcast link to each room assigned to this MLCT andundedicates any remaining broadcast links. Thereafter, control is passedto operational block 6702. If the test result of conditional branchpoint 6716 is false, control is transferred to conditional branch point6718.

Conditional branch point 6718 tests to determine if the number ofbroadcast links is sufficient for dedicating the links to the roomsassigned to other MLCTs. If the test result is true, operational block6719 dedicates a broadcast link to each room served by another MLCT andundedicates any remaining broadcast links. Thereafter, control is passedto operational block 6702. If the test result of conditional branchpoint 6718 is false operational block 6720 sends a message to the MS 101indicating the number of broadcast links is insufficient. Thereafter,control is passed to operational block 6702.

What is claimed is:
 1. Apparatus for transmitting video signals to aplurality of remote locations including,means for controllably selectingvideo information supplied by said locations to be transmitted to othersof said locations, and means for controllably adding or deleting saidremote locations to or from a conference comprising means for monitoringinformation transmission from said locations to determine ifsatisfactory transmission exists, and means for inhibiting selection ofvideo information from any one of said locations when transmission fromthe location is unsatisfactory.
 2. The apparatus as defined in claim 1wherein the information from the remote locations includes audioinformation and wherein said monitoring means includes means formonitoring the audio information received from the locations todetermine whether the audio information is satisfactory.
 3. Theapparatus as defined in claim 2 wherein the information from the remotelocations further includes control channel information and wherein saidmonitoring means further includes means for monitoring the controlchannel information from the remote locations to determine whether goodcontrol channel communications exists and said inhibiting means includesmeans responsive to outputs from said monitoring means for inhibitingselection of the video information from a location when the audioinformation from the location is unsatisfactory or the control channelcommunication from the location is not good.
 4. The apparatus as definedin claim 1 further including a plurality of communications ports fortransmitting and receiving information from the remote locations, andwherein said means for adding or deleting locations further includesmeans for controllably assigning said communications ports to saidremote locations.
 5. The apparatus as defined in claim 4 wherein saidinhibiting means further is responsive to outputs from said assigningmeans for inhibiting selection of the video information from thecommunications ports which have not been assigned locations in theconference.
 6. In apparatus for transmitting video signals to aplurality of remote locations,means for dynamically adding and deletingconference locations from a conference, wherein each of the conferencelocations generates audio information and control channel information,said adding and deleting means including means for monitoring audioinformation received from the conference locations to determine whetherthe audio information is satisfactory, means for monitoring controlchannel information from the conference locations to determine whethergood control channel communication exists, and means responsive tooutputs from said audio monitoring means and said control channelmonitoring means for controllably inhibiting selection of a video signalfrom a conference location for transmission to any of the otherconference locations when the control channel communication isdetermined to be not good or when the audio information from thelocation is determined to be unsatisfactory.
 7. The apparatus as definedin claim 6 further including means for controllably mixing audioinformation from said conference locations, and means responsive to anoutput from said audio monitoring means for controllably muting theaudio information from a conference location from being mixed with audioinformation from other conference locations when the audio informationfrom the conference location is determined to be unsatisfactory.
 8. Theapparatus as defined in claim 6 wherein the audio information from eachof the conference locations is digitally encoded and includes aprescribed parity check bit, and wherein said audio monitoring meansincludes means responsive to the received audio information forregenerating a parity check bit and means for comparing the parity checkbit received from the location and the regenerated parity check bit togenerate said monitoring means output representing whether said audioinformation is satisfactory or unsatisfactory.
 9. The apparatus asdefined in claim 6 wherein said control channel information includes apredetermined sequence of command messages to be transmitted to theconference locations and said control channel monitoring means includesmeans for verifying responses from the conference locations to saidsequence of command messages for determining whether the good controlchannel communication exists.
 10. The apparatus as defined in claim 6wherein each of said conference locations generates talker detectionmessage signals, and the apparatus further includes means responsive tosaid talker detection message signals from the conference locations forresolving contention for the video signal to be transmitted to otherconference locations by selecting one of the conference locationsgenerating a talker detection message signal with an absence of talkerdetection message signals from the other locations during apredetermined talker timing interval, and wherein said inhibiting meansinhibits said talker detection message signals from being supplied tosaid talker contention resolving means when said control channelcommunication from the conference location is not good or said audioinformation from the conference location is unsatisfactory.
 11. Theapparatus as defined in claim 6 wherein each of said conferencelocations generates graphics video transmission request message signals,and the apparatus further includes means responsive to said graphicsvideo transmission request message signals from the conference locationsfor resolving cotention to determine which graphics video signal is tobe transmitted from a conference location to the other conferencelocations in the conference, and wherein said inhibiting means inhibitssaid graphic transmission request message signals from being supplied tosaid graphics contention resolving means when the control channelcommunications to the location is not good.
 12. The apparatus as definedin claim 6 wherein each of said conference locations can generate avideo select request signal, and the apparatus further includes meansresponsive to video select request message signals from the conferencelocations for selecting the video signal to be transmitted to individualones of the conference locations from the other locations, and whereinsaid inhibiting means inhibits said video select request message signalsfrom being supplied to said video selecting means when the controlchannel communication from corresponding ones of the conferencelocations is not good.
 13. The apparatus as defined in claim 6 whereineach of said conference locations can generate a video select requestsignal, and the apparatus further includes means responsive to videoselect request message signals from the conference locations forselecting the video signal to be transmitted to each conference locationfrom a selected one of said conference locations, and wherein saidinhibiting means inhibits said video select request message signals frombeing supplied to said video selecting means when the control channelcommunication from the selected location is not good.
 14. Apparatus asdefined in claim 6 further including a plurality of communications portsfor transmitting and receiving information from said remote locations,and means for controllably assigning said communications ports to saidlocations, and wherein said inhibiting means is further responsive tooutputs from said assigning means to inhibit selection of videoinformation from ones of said communications ports that have not beenassigned to a location.
 15. Apparatus as defined in claim 14 furtherincluding means for controllably mixing audio information from theconference locations, and wherein said assigning means enables saidmixing means to mix the audio information from conference locationsassociated with assigned ports and inhibits said mixing means frommixing the audio information from other locations.
 16. The apparatus asdefined in claim 15 wherein said assigning means includes means foridentifying the type of location associated with the assigned ports asbeing a conference location or as being a location associated withanother apparatus for transmitting video signals to a plurality ofremote locations, and said mixing means includes means responsive to anoutput from said identifying means for supplying mixed audio from alllocations except a conference location associated with a porttransmitting the mixed audio information and for supplying mixed audioinformation from ports assigned only to conference locations to portsassigned to said other apparatus.
 17. The apparatus as defined in claim14 wherein said remote locations include conference locations and caninclude others of the apparatus for transmitting video signals to aplurality of remote locations, each of said communication ports in theapparatus includes incoming and outgoing transmission paths, and whereinsaid port assigning means further includes means for assigningcommunication ports to conference locations associated with thisapparatus, means for assigning at least one outgoing transmission pathto said others of the apparatus having at least one conference locationto be connected in a conference and means for assigning at least oneincoming transmission path to each of said others of the apparatushaving at least one conference location to be connected in theconference.
 18. The apparatus as defined in claim 17 wherein a number ofoutgoing transmission paths assigned to said others of the apparatus isdetermined in accordance with a first predetermined relationshipdependent on the number of conference locations assigned to thisapparatus and the total number of conference locations assigned to saidothers of the apparatus.
 19. The apparatus as defined in claim 18wherein said first predetermined relationship is the minimum of thenumber of conference locations assigned to this apparatus or the totalnumber of conference locations assigned to said other apparatus.
 20. Theapparatus as defined in claim 18 wherein said assigning means furtherincludes means for dedicating said assigned outgoing transmission pathsto conference locations in accordance with a second predeterminedrelationship dependent on the number of conference locations assigned tothis apparatus and the total number of conference locations assigned tosaid other apparatus.
 21. The apparatus as defined in claim 20 whereinsaid second predetermined relationship includes determining if thenumber of outgoing transmission paths is equal to or greater than thenumber of conference locations assigned to this apparatus, if so,dedicating an outgoing transmission path to each of the conferencelocations assigned to this apparatus, if not, determining whether thenumber of outgoing transmission paths is equal to or greater than thetotal number of conference locations assigned to said others of theapparatus, if so, dedicating an outgoing transmission path to each ofthe conference locations assigned to said others of the apparatus. 22.The apparatus as defined in claim 17 wherein said port assigning meansfurther includes means for storing information identifying conferencelocations assigned ports in this apparatus and conference locationsassigned ports in said others of the apparatus, if any, having at leastone conference location assigned to the conference, said storedidentifying information including a number of outgoing transmissionpaths assigned to said others of the apparatus, the number of conferencelocations assigned to this apparatus and the total number of conferencelocations assigned to said others of the apparatus, and means fordynamically dedicating signal transmission on the outgoing transmissionpaths to conference locations assigned to this apparatus in accordancewith a prescribed relationship dependent on said stored identifyinginformation.
 23. The apparatus as defined in claim 22 further includingmeans for dynamically transmitting to said other apparatus the identityof conference locations assigned ports in this apparatus.
 24. Theapparatus as defined in claim 22 wherein said prescribed dedicatingrelationship includes determining whether the stored number of outgoingtransmission paths is equal to or greater than the stored number ofconference locations assigned to this apparatus, and if so, dedicatingan outgoing transmission path to each of the conference locationsassigned to this apparatus, and if not, determining whether the storednumber of outgoing transmission paths is equal to or greater than thetotal number of conference locations assigned to said others of theapparatus, and if so, dedicating an outgoing transmission path to eachof the said conference locations assigned to said others of theapparatus.
 25. The apparatus as defined in claim 24 wherein said portassigning means further includes means for transmitting to said othersof the apparatus information indicating the dedication of said outgoingtransmission paths.
 26. The apparatus as defined in claim 25 whereinsaid port assigning apparatus further includes means responsive to saidoutgoing transmission path dedication information for identifyingincoming transmission path dedication.